Translating the Words: Multiclassing
First, a warning. Truenamers are fundamentally casters, and we all know that losing caster levels is bad. That said, just how bad it is really depends on what level you are. A lot of the higher-level utterances don't really stick out as must-haves, so honestly, it's a lot easier to give up 6th level utterances than it is to give up 9th level spells. Losing a single level does mean that you won't get Conjunctive Gate, but unless you're going to hit level 20, that's not an issue. What might be an issue is not getting Speak Unto the Masses. If you think the game might hit level 17, you'll want to think long and hard about losing Truenamer levels.
Factotum: Hailing from Dungeonscape, a dip into Factotum (with Able Learner, ideally) is as good for a Truenamer as it is for everyone else, since you'll finally have something to spend all your skill points on. You can even afford to do this at 1st level, since you won't have to buy Truespeak cross-class. You won't get much use out of the Inspiration abilities with only 1-3 levels of Factotum, and the 1/day skill boost from Cunning Knowledge is tied to your Factotum level (so don't think it makes you autosucceed on a Truespeak check once per day), but it's something. Take either 1 level or 3 . . . 1 level opens up the skills (with Able Learner) and a little bit of use for your INT, and 3 levels gets you the delicious Brains over Brawn ability. Too much more than that, though, and you're pretty much more Factotum than Truenamer.
Rogue: Like Factotum, only not quite as good. You'd dip Rogue for the same reason you'd dip Factotum: to open up additional skills with Able Learner. Factotum does it better, unfortunately.
Marshal: This class comes to us from the Miniatures Handbook. What you want out of it is the Motivate Intelligence aura, which lets you add your CHA mod as a circumstance bonus to any INT-based skills you use, including Truespeak and Knowledge. If you have a good CHA anyway (for UMD or utterance save DCs) and you're desperate for bonuses, this might be worth it. If you don't have especially good stats, though, this probably isn't worth the level. It does at least give you armor proficiency, though, so there's that.
Incarnate: From Magic of Incarnum, the Incarnate does absolutely nothing* to advance your Truenaming, but even a single level in it can drastically increase your out-of-combat options if you know what you're doing. Not for everyone, and don't go too deep lest you forget to come out, but having a couple soulmelds never hurt anyone. * Okay, technically, the Lucky Dice can give you a +1 bonus on Truespeak checks, but it costs your swift action every round, making it such a bad deal that I don't even really consider it to be there at all.
Exemplar: There's a little bit of controversy about this Complete Adventurer PrC. I am officially of the opinion that Skill Mastery does not apply to Truespeak for the same reason that it doesn't apply to UMD (it's not stress or distraction that's preventing you from taking 10; it's a clause in the skill itself), but hey, you might apply it to something else. That said, you'll still get a quick +4 competence bonus, which can be worth it if you don't have a competence item. Whether that's better than a new utterance and another step along the path in your primary class is up to you, and basically depends on how much you like high-level utterances and how desperate you are for bonuses. This class isn't hard to qualify for, at least. (A note about Skill Mastery: if your GM believes that Skill Mastery lets you take 10 on Truespeak, it might be worth it to take a look at the feat Hardened Criminal on pg. 95 of City of Stormreach, which has language that's even looser than Skill Mastery and is available for just two feats. Now, whether two feats is cheaper than one level is an open question, but Hardened Criminal comes online at a MUCH lower level than Exemplar. Either way, check it out. If you have a tolerant GM and a taste for CharOp tricks, the prereq feat is Iron Will, which you can buy from the Otyugh Hole.)
Uncanny Trickster: Coming from Complete Scoundrel, this is one of the only two PrCs that advances utterances (the other is Legacy Champion, and this gives you more out of the deal). This class is 3 levels long, and the last 2 advance all your Truenamer features, including utterances. In exchange, you'll get a greatly expanded skill list with way more skill points (even though you have to buy Truespeak cross-class, unless you have Able Learner), a few levels with a good Reflex save, and some bonuses to working with skill tricks. The downside is that it's hard to make a Truenamer qualify for a lot of skill tricks (you're pretty much getting Collector of Stories and that's it), and you need to know four skill tricks to get in. This is much easier if you took a dip in Factotum with Able Learner, of course.
Asking Nicely: Not-Quite-RAW PrCs
If your GM is very nice, you might be able to convince him or her to let a few PrCs that are especially thematically appropriate advance Truenaming, even though they don't do so by RAW. Since this isn't RAW, I won't spend too long here, but you might consider asking.
Paragnostic Apostle: You can qualify for this Complete Champion PrC by RAW, but you can't do much with it. You'll get a slightly expanded skill list (though Truespeak will be cross-class for you), a Bardic Knowledge-style effect, and a few little bonuses that mostly won't apply to your utterances. It does advance your standing with the Assembly (which is the main benefit), and a couple of the Knowledge Is Power boosts are acceptable, but this is more a thematic choice than a powerful one.
Human Paragon: The Paragon classes from Unearthed Arcana aren't quite base classes and aren't quite prestige classes, but I didn't want to devote an entire section to it. You can easily qualify (the only prereq is basically to be a human, though ask if having the [human] subtype will get an illumian in the door). This is 3 levels long, 2 of which advance casting. You'll get to choose your own skill list (so you can get a few levels of actually taking other skills), you'll get a permanent class skill (so you can expand your Truenamer list a bit . . . or get Truespeak always in-class and dip away, but whatever), you'll get a bonus feat, and you'll get a stat boost, which will probably be INT. Worth a lost Truenamer level? You tell me. Decent choice if your GM allows it, though.
Urban Savant: Tucked away on page 100 of Cityscape, this PrC has 9/10 casting advancement, loses the caster level at the END (so you can just take 9 levels and be happy), and has a really cool knowledge focus. You already have crazy high Knowledge checks, and this lets you do something with them. You'll have to convince your GM that the Paragnostic Assembly is close enough to the League of Eyes, but if you're even considering taking this class at all, your GM's willing to work with you. You'll get an expanded skill list (even taking cross-class Truespeak, you're coming out ahead), you'll get something to do with your Knowledge checks, and you'll get a good Reflex save. Decent enough.
Dialects of the Words: Truenaming PrCs
Weirdly enough, none of the PrCs in Tome of Magic advance utterances. Not one. Yeah. They're not really made with Truenamers in mind, it would seem . . . nor are they written with anyone else in mind, really.
Acolyte of the Ego:
Entry: Easy. You basically can't not qualify, at least if you have any sort of inclination towards Truenaming.
I think this is meant to be the self-buffing gish Truenamer (full BAB, good Fort? Huh). Basically, you get a bunch of utterances-that-aren't-called-utterances that only affect you. Most of them aren't that good . . . the best is probably the one that lets you use Dimension Door. Nothing else really stands out, though the insight bonuses to STR/DEX/CON are at least interesting, if not particularly worth it. Most of the cadences (for so they are called) aren't as good as utterances, and that's sad. A Truenamer might consider dropping in for two levels to get Cadence of the Distant Step, but no more than that.
Entry: Easy/Medium. If you can find someone who can cast the Ritual of Renaming, this is trivial, though a bit late. If no one in your GM's world gives three bits of a damn about Truenaming, this is harder.
This class is basically too little, too late. The abilities you get are not that good, come online way too late, and have a really bad save DC (10 + class level + INT . . . on a class that's 5 levels long and that can't be entered before ECL 11. Oops). Also, too many things are based on your class level, which, as indicated, will not be much. The abilities are flavorful, but none of them are actually worth using. This class would be worth rebuilding from the ground up, since it's a neat concept, but it fails at everything it tries to do. (Fun fact: The sample Bereft has 10 levels of Fighter. Don't ask me.)
Entry: Hard. You need at least 4th level Cleric spells, basically.
So, you need some nasty prereqs, and you're giving up 4 levels of Cleric casting. In exchange, you get some summons that you could probably get with Planar Ally anyway, as well as a really weak breath weapon. Um, yeah, pass. (I also find it weird that the Heavenly Entreaties are bound by both a daily limit AND the Law of Resistance. Harsh.)
Disciple of the Word:
Entry: Medium. A Monk with an interest in Truespeak will qualify pretty easily, but still, not like Monks have skill points to spare.
Yo dawg, I heard you like poorly designed classes, so we put a poorly designed class in your poorly designed class so you can fail to contribute while you fail to contribute!
Sorry. I've been waiting to do that. Anyway, yeah, this is about what you can expect from a class that blends Monk and Truenamer. Now your Monk can only dump CHA! Wheeeee! Anyway, this class both explicitly does and explicitly does not advance flurry of blows penalty, so I think it's actually like a riddle or a koan or something. Or maybe it's just bad writing. Anyway, ask your GM. Overall, you get a lot of “Spend a use of Stunning Fist and make a DC X Truespeak check to do Y.” Some of them are OK, but none of them are great. I do like that it's a DC 45 Truespeak check to overcome DR/epic. Probably not worth it.
Entry: Hard. This requires you to have 4th level spells from a real casting class.
Basically, you can permanently bind fiends by spending gold, but since they'll refuse to take any actions you don't order them to, you can never break the action economy with them. For a design goal, this is good. Coming into the 3.5 milieu, this is bad, since everything else that summons breaks the action economy, and this doesn't get much to make up for it. As you advance in the class, you get different fiends to bind, though you lose 3 caster levels. At class level 9, you do get the ability to issue two commands at once, but it's probably not worth it. This class is interesting and fluffy, but don't try it in a high-op group, or you'll fall behind.