Hi, I'm looking for inventive ways to boost the DC of spells, particularly Enchantment/mind affecting spells. There's the obvious choice of Spell Focus, but what else is out there, in the way of feats, races, classes and items?
Other than the obvious -- Spell Focus and Greater Spell Focus -- I think your only option are the two fey feats in Complete Mage. The first is the entry feat and the second adds one to all Enchantment DCs.
Take the Shadow Weave Magic feat (Player's Guide to Faerun) and a level in Shadow Adept (gets you Insidious Magic, Pernicious Magic, Tenacious Magic). Gets you +1 to the DCs of all illusion and enchanment and necromancy spells, and several other nifty bonuses.
A Master Specialist (Complete Mage) can add to the DC of spells cast from a specialty school.
The Fey Heritage Feat followed by Fey Power feat may help I seem to recall.
I think the 10th level of Wild Soul giving a +1 to save DCs for Enchantments. A wild soul
A single level of Mindbender Prestige Class is probably a good flavor (and power) for this character. Combined with the Mindsight from page 126 of the Lords of Madness book, and you will have a powerful enchanter that can pinpoint opponents very well.
The Wild Mage prestige class + the Practices Spellcaster feat might help with this.
A veil of allure (Magic Item Compendium) is good for this character. +2 to the DC of enchantment and charisma based extraordinary/supernatural abilities for 14,000 gold.
I think there's also a reserve feat in Complete Mage that gives a +1 bonus to enchantment spells, along with allowing you to do something (distract foes?) if you have a spell in reserve.
There's another one that allows you to give the target of your enchantment spells a small penalty (I believe to AC) even if they fail their save.
Otherwise just boost your main casting stat.
If you're looking for situational advantages, it's arguable that an unseen, silent/telepathic enchanter would not incur the -5 penalty for "threatening" or "having allies that threaten" a foe when trying to charm someone. That's specific to a certain spell type, but worth mentioning.
Binders from the Tome of Magic can bind a vestige which improves all your spell save DCs by 2. It's not a first level vestige though, you'd need some investment in the class to do it.