Improving charisma checks

[Nebuul]

Is there any way for a chaotic neutral character to increase charisma checks other than increasing their charisma score? For example, for a charisma-based warlock who just learned the Charm invocation, is there anything to help out with the opposed charisma checks for forcing opposed tasks via charm monster?

[RoboEmperor]

Is there any way for a chaotic neutral character to increase charisma checks other than increasing their charisma score? For example, for a charisma-based warlock who just learned the Charm invocation, is there anything to help out with the opposed charisma checks for forcing opposed tasks via charm monster?

Take a dip in martial.

Get access to the spell Surge of Fortune.

Stock up on Luck Blades (d20srd)

[Gullintanni]

The Admirals's Bicorne (Stormwrack) and Circlet of Persuasion (SRD) come to mind.

The former provides a +5 bonus to Cha checks, and some other, not insubstantial bonuses. It clocks in at 51 000gp though, so it's not an early game item.

The Circlet of Persuasion offers +3 to cha checks, for only 4500gp. Much more affordable.

[Demidos]

Take a dip in marshal.

Get access to the spell Surge of Fortune.

Stock up on Luck Blades (d20srd)

Fixed that for you.

[RoboEmperor]

Fixed that for you.

Thanks.

There's also Moment of Prescience.

If you're a warlock you just need to keep recasting charm on that one creature forever and ever until you succeed the check. Restrain him somehow while you do it, and once you get your Imbue item class feature start making scrolls of Surge of Fortune or Moment of Prescience, or take a Marshal Dip, or invest in rerolls which is what luck blades give. Circlet of Persuasion is a fine choice.

[Zaq]

Illumians with the Vaul power sigil and 2 or more HD have a +2 on all CHA-based checks. (Technically, if you have Assume Supernatural Ability, you can Alter Self into an illumian and gain that benefit, but that's almost never going to be cost-efficient, especially with the "-2 to everything" clause in Assume Supernatural Ability that everyone seems to forget about and that I don't know a one-and-done way around. Better to just be an illumian in the first place, though it sounds like you've already started playing the character.)

It costs a garbage prereq feat (Negotiator, one of the awful +2/+2 skill feats from the PHB), but the feat Charming (Dragon Compendium, pg. 93) does exactly what you're looking for: +3 typeless bonus on CHA checks to exert control over mind-controlled targets. Maybe you can find something else with Negotiator as a prereq to at least make it be a prereq feat that opens up multiple other interesting options rather than simply being a tax feat that must be lit on fire?

[Biffoniacus_Furiou]

Surge of Fortune (Cleric 5, Complete Champion): +2 luck bonus on attack rolls, saving throws, skill checks, ability checks, spell penetration checks, and AC for 1 round/level. You can use an immediate action to discharge the spell in a moment of perfect fortune, making your next attack roll, saving throw, skill check, ability check, or spell penetration check treated as a natural 20.

Marshal (http://archive.wizards.com/default.asp?x=dnd/ex/20030906b) 1: Motivate Charisma adds Cha bonus (again) to Cha checks.
Competition domain: +1 on all opposed checks.
Charm domain: +4 Cha (untyped) once per day for 1 minute as a free action.
Improvisation (Bard 1, SC): Gain a bonus pool of +2 per caster level to be divided among attack rolls, skill checks, and ability checks for 1 round/level, can't add more than half your caster level (1/4 the total bonus pool) on any one check.
Ray of Hope (Bard 1, Cleric 1, BoED, [Good]): One living creature (any alignment) gets a +2 morale bonus on saving throws, attack rolls, ability checks, and skill checks for 1 round/level.
Circlet of Persuasion: Already mentioned, gives +3 to all Cha-based checks at a price of (bonus squared) x 500 gp. Ask your DM if you can get/make a more powerful version.

I don't know if you're going to want to dip Marshal or Cleric for any of that, but a Wand of Improvisation (or an Eternal Wand even) with a decent caster level can give you a pretty good bonus to that and many other things. Surge of Fortune looks like it would be perfect but being a 5th level spell it's just out of reach for a wand, and is going to be particularly expensive regardless.

[Seharvepernfan]

stone of good luck
pale green prism ioun stone
circlet of persuasion
good hope spell

that's all I know of from core

[Troacctid]

I was going to recommend Nymph's Kiss, but I guess that's off the table if you're Chaotic Neutral. (Maybe it'll be useful for future readers with the same question who Google up this thread, though. Shout-out to the Google folks.)

The Lucky Dice soulmeld can give you a +1 luck bonus on all skill and ability checks at will as a swift action. It can also give bonuses to attack rolls, damage rolls, and saving throws, so it is quite powerful in general in addition to being useful for this purpose.

[Thurbane]

Touchstone (City of the Dead) feat (Sandstorm) gives +1 insight bonus to all Charisma-based checks as it's base ability.

[RoboEmperor]

That reminds me. Domain.

Lust Domain gives you +1/clericlevel cha for 1 round.
Charm Domain gives you +4 untyped bonus to cha.

[skunk3]

You could also invest in items that allow you to read/detect thoughts, as well as items that disguise your alignment to get a better read on the person and prevent them from reading you. A mask of lies wouldn't help boost your CHA but it could possibly help indirectly.

[Nebuul]

Thanks for all the suggestions!