And now the continuation of the PrC's worthy of specific mention (waht out they are heavily debated)

Legacy Champion
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requirements: knowledge history 5 ranks, the least legacy feat, ownership of a legacy item, character level 10'th
What does it do for a warlock? Well, consider the fact that you get +2d6 hellfire damage from Hellfire warlock. Legacy champion gives you:
At each level except 1st and 7th,
you gain class features and an increase in effective
level as if you had also gained a level in a class to which
you belonged before adding the prestige class level.
The specific class features you gain include spells
per day (and spells known, if applicable), improved
chance of turning or destroying undead, metamagic or item creation feats, bonus feats, monk special abilities,
sneak attack progressions, and so on, depending
on the class. You do not, however, gain the benefit of
your previous class’s Hit Dice, attack progression,
skill points, or saving throws. If you had more than
one class before becoming a legacy champion, you
must decide to which class to add each
The trick is debatable becuase of the "As if you had taken another level of that class" part. Some argue that it continues HFW's Hellfire blast (which explicitly gives +2d6 of hellfire damage) and invocations as if you had gained a level, but without gaining that level and not going beyond the 3 levels of HFW. The people against that say that you can't gain levels beyond the 3rd and thus not use it to gain benefits. Check wiht your DM if this is allowed as a tactic. If it is you get a nice powerboost, if it isn't, you don't


Uncanny Trickster
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Requirements: 4 skills with at least 8 ranks, 4 skilltricks.
what does it do? It does that same as Legacy champion:
At each level after 1st, you gain class features (including spellcasting ability) and an increase in effective level as if you had also gained a level in a class to which you belonged before adding the prestige class level. You do not, however, gain the benefit of your previous class’s Hit Dice, attack progression, skill points, or saving throws. If you had more than one class before becoming an uncanny trickster, you must decide to which class to ad
the trick is equally hot debated as Legacy Champion and you must again ask your DM. (some tricks are worth it however)



then a list of useful tricks on your warlock (if you have the skillpoints to spare):
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Tricks you love, separated by role:

face:
  • Second Impression
  • Assume Quirk
  • Never Outnumbered (max intimidate)

these tricks increase your chances of succeeding at social skills by offering second chances or expanding on the uses of the skills. Very useful shoudl you be a face (and that without investing in face invocations or feats)
scouting:
  • Clarity of Vision (instead of the invoaction see the unseen)

or 1 round you can see invisible creatures within 30 feet if you can succeed on a dc 20 spot check. Coupled with a wand of gliterdust no more invisibility for the enemy, no invocation requiered and everyone enjoys it.
casting:
False Theurgy: this is debatable, but worth trying your DM for. It is uncaertain if by spells invocations are also allowed.

Miscellanious:
Easy escape: if you get into melee you will need some form of free movement, this trick gives it: you can escape from a grapple using this trick.

Skilltricks are a great way of enhancing a character with minute resources (only 2 skillpoints for a trick). The rest of the skilltricks are either too situational or not helpful at all.