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Don't forget heighten spell...if that 20th level mage has the heighten spell feat, the Charm Person DC can be 23 if he casts it as a ninth level spell...or as high as 25 if he has Spell Focus: Enchantment.
Well, Wizards of the Coast actually published an article on min/maxing (minimizing a characters weaknesses, and capitolizing on his strong suits) that was fairly informative, if basic.
Dragon Magazine has been doing little blurbs called Power Plays for quite some time using rudimentary abilities in strong tactical combinations to improve their effectiveness.
And there is a glut of clever and detailed analysis and theories on how to use various class abilities, feats, spells, and magical items to their utmost capability.
For some examples of what kind of mindset to be in for high powered gaming, try checking out this thread.