Conjugating the Words: Feats
There are a few feats that you're going to be taking. This is not negotiable. The first, as is probably obvious, is Skill Focus: Truespeak. Nothing fancy here, folks, just a +3 to the skill that makes you tick. Feat tax? Yup. You're taking it anyway. Probably at level 1, unless there's some really specific first-level-only feat you want (and even then, you'll probably wish you had SF: Truespeak).
The second feat is Quicken Utterance. You'll take this feat at level 9. Why? Because it's one of the only things that makes a Truenamer, well, almost decent.
Now, the feat is absurd on the face of it, and it really points out some of the problems with the design of the class. The DC is increased by 20. That means that in order to have any chance at all of succeeding at a Quickened utterance, you had to have had a greater than 100% chance of success before you Quickened it. Think about that for a minute. They printed this feat being fully aware that you needed to have that kind of Truespeak mod. That says something about this whole mess, and I'm not sure how I feel about that. Anyway, you're an optimizer. You can get a +20 above baseline if you really want one, right? Of course you can, one way or another. Even if you can't get yourself up to auto-success levels with Quicken, you lose nothing but your swift action for failing (and you'll notice how few native uses for your swift actions you have). Still, throwing out two of your highest-level effects per round is pretty nice. See, that's the thing . . . the DCs of utterances are based on the target, not on the utterance. Casting Minor Word of Nurturing (a level 1 utterance) on your friend is exactly as hard as casting Greater Word of Nurturing (a level 6 utterance) on that same friend. This means that as soon as you can get Quicken, you can Quicken your best utterances just as easily as your weakest ones. That's a trick that very few classes can replicate, which is nifty in its own right. Either way, assuming that you can get your Truespeak mod up to the stupidly high levels this requires, this is an immeasurably valuable feat.
Almost every Truenamer will also take Extend Utterance, just because the duration on your utterances is so bloody short. Now, you won't want to Extend every utterance you speak since the Law of Sequence (may it rot in Hell) will get in your way, but it's definitely an option you'll want in your toolkit sooner or later.
There are also a lot of feats that affect SLAs, such as SLA Focus (RoF), Boost SLA (BoVD), Empower SLA (MM), Quicken SLA (MM), Maximize SLA (CArc), and others. Since utterances are SLAs, they apply. Most of them aren't very good, since they have really harsh limits on how often you can use them and which SLAs are valid targets, but they're there. Be sure to look closely at whether it affects one SLA chosen when you take the feat, or if it affects any SLA you have.
Anyway, here are a few feats that you might want to look into. They won't all be appropriate for every Truenamer, but they're some options.
Knowledge Devotion: (Complete Champion) You have a lot of Knowledge skills. You have an INT focus. You have ways to boost Knowledge skills and reasons to do so. If you ever want to use an actual weapon (even a crossbow in the back), this is for you.
Wild Cohort: (Online) Definitely not for everyone, but this is useful for two reasons. One, it gives you a backup thing to do in combat (even if you rolled poorly on your Truespeak check or if all your useful utterances are tied up by the Law of Sequence, your pet can still eat something), and two, pets have low CR, so they tend to be pretty easy to affect with Truespeak. Handle Animal isn't a class skill, but if you stick to the basics, you should be OK with a few cross-class ranks (and perhaps Universal Aptitude).
Mortalbane: (Book of Vile Darkness) If you're into directly damaging utterances, this is a decent damage bump, especially at low levels. Some GMs aren't too fond of it, but it's legit. This is also cool because it should trigger on every round of an utterance that isn't instantaneous (Energy Negation, for instance), allowing those dice to add up. Note that despite the name and the book, there's nothing inherently evil about this feat.
Boost SLA: (Book of Vile Darkness) If you're going to be taking feats to bump up the save DCs of your utterances, this one is probably the best, since it works on ALL your utterances, and it works three times per day per utterance. You're not going to find a better deal than that.
Insightful Reflexes: (Complete Adventurer) As a purely defensive feat, this isn't going to be very high on your priority list, but since your INT is likely to be much higher than your DEX, this can make you very difficult to hit with Reflex saves.
Improved Initiative: (PHB) You're functionally a caster-type kind of fellow, and you've got a debuff or two up your sleeve. Going first for you is as nice as it is for anyone else. Worth a feat? Your call, my friend, but you'll probably at least notice this.
Enhanced Power Sigils: (Races of Destiny, illumians only) The benefits of this feat are marginal at best, but if you're an illumian who's desperate for another +1 to Truespeak, this will give you some other bonuses as well, including to Knowledge checks. Not a top-tier choice by any stretch, make no mistake.
Make Your Own Luck: (Complete Scoundrel) Getting a bunch of rerolls is a decent way of making Truespeak more reliable, if you don't have any other ways of getting your check up high. You'll need other luck feats as well (one as a prereq, and others to get extra rerolls). Note that rerolling with this feat is an immediate action, so you can't Quicken in the same turn unless you blow a level on Fortune's Friend. If you go this route, I recommend Lucky Start as your prereq and Unbelievable Luck as a follow-up.
Trivial Knowledge: (Races of Stone, gnomes only) If you're really dedicated to the cause of being a know-it-all, this might make being a gnome worth looking at.
Favored: Paragnostic Assembly: (Cityscape) Assuming that you are in fact associated with the Assembly (see below), this can probably snare you a +2 competence bonus on Truespeak. Talk to your GM about having it increase your standing, too.
Focused Skill User: (Complete Psionic) If you're psionic (perhaps from taking Hidden Talent or from being a race with built-in PP), you can get a +2 bonus to any three skills of your choice while focused, and yup, that includes Truespeak. It's a competence bonus, so if your GM lets you get a competence item, this won't stack, but it's there if you really need another +2 just that badly. Probably not worth the investment (one feat for Skill Focus is one thing, but this costs either two feats or a feat and a race, and that kinda hurts), but it exists.
Skill Focus: Knowledge: (PHB) Hardly overwhelming, but it might bump you up to the next level in the Paragnostic Assembly (see below).
(Minor) Utterance of the Evolving Mind: (Tome of Magic) Part of the problem with Truenaming is that there aren't enough good utterances in the first place, so you'll only rarely want an extra one (especially at the cost of a feat), but this does have a use or two. If you started at low enough levels (say, level 1) that you wanted to take an utterance that starts out OK and really doesn't help at later levels, this can make that sting a little bit less.
Focused Lexicon: (Tome of Magic) I'm not actually recommending this feat. I'm just pointing out something stupid, because we clearly haven't seen enough of that yet. I believe that this is intended to be similar to Spell Focus, but as written, what it does is make the Truespeak DCs harder, not the save DCs. In other words, you're paying a feat to make life harder for yourself, with no direct benefit. RAI? I sure as hell hope not. RAW? Looks that way. (If you play this by what I believe RAI to be, i.e., that it works to increase the save DCs of your utterances against your chosen group, it's decent enough if you really have that many utterances that allow saves AND if you know what kind of enemies you're going to be facing, but it's probably not worth it overall.)
Ancestral Relic: (Book of Exalted Deeds) If your GM is very stingy with the magic marts, this is pretty much a guaranteed way to get an Amulet of the Silver Tongue, assuming you get treasure at all. If your GM will let you buy an AotST normally, this isn't worth it.
Able Learner: (Races of Destiny) This makes dips out of Truenamer easier, it lets you take some skills that you wouldn't want to take otherwise (and Universal Aptitude can do a lot to make up the gap), and when combined with a dip in Rogue or Factotum, it can give you something to do with all those skill points you have. It's limited to humans only, but since illumians are humanoids with the [human] subtype, you can make a pretty strong argument that they qualify. The only downside is that this feat must be taken at level 1, which means you might not get Skill Focus until later. To be honest, I could see an argument for taking this even on a build that's just Truenamer 20. Just because you couldn't max the skills doesn't mean they're not worth investing in, especially if you get bonuses from another source, like Universal Aptitude or an illumian sigil.
Precognitive Visions: (Fiendish Codex I) A +1 insight bonus for the cost of two feats is about the worst deal I can find, but in the interest of completion, this CAN give you an always-on bonus to Truespeak. I recommend . . . not doing so.
Chosen of Evil: (Elder Evils) Take 1 CON damage to gain an insight bonus equal to the number of Vile feats you have on one of any number of checks, including skill checks. No use limit other than your CON (and I'm sure we all know how to get around that), but it eats your immediate action, which means no Quickening. If you're into Vile feats that eat your immediate action but can give you Truespeak bonuses, check out Master's Will, also from Elder Evils. Nasty prereqs, though. Neither comes highly recommended.
Piddly Bonuses: (Varies) There are a lot of feats out there that can give you a +1 or +2 on a bunch of different checks a limited number of times per day. These are pretty much uniformly terrible unless you're using them for prereqs (and even then, I don't know of anything that uses these things as prereqs that's actually worth it), but just for completion's sake, here's as many as I could find: Devil's Favor (FCII; +2 untyped to any check 1/day per Devil-Touched feat); Heroic Destiny (RoD; +1d6 untyped to seriously almost any check 1/day); Favored of the Companions/Knight of Stars/Servant of the Heavens (BoED; +1 luck to any check 1/day while performing an act of good); Disciple of Darkness/Thrall to Demon (BoVD; +1 luck to any check 1/day while performing an act of evil). There may be others, but the point is, they're all terrible. Don't do this to yourself.