The non-physical abilities are usually tricky to roleplay so just avoid min/maxing and put 10 (or more) points in int/wis/cha and you'll be fine. "8 charisma or 10 charisma" might feel like a small difference on the paper, but in reality (erh.. game reality, that is!) it's much, much bigger. 10 is "average Joe" (as you probably know) while 8 puts you to the same level as "Gnoll, Dire Boar, Manticore, Gorgon". And 6 charisma... Heeeh! Either others would be running from your mere presence (because of your unearthy ugliness or rudeness) or you would be running from them (your "negative" confidence). :P
Taken from the D&D 3.5 handbook:
"Charisma measures a character's force of personality, persuasiveness, personal magnetism, ability to lead and physical attractiveness. This ability represents actual strength of personality, not merely how one is perceived by others in a social setting."
So, that people trust the CHA 10 cleric but not the CHA 18 pirate is completely valid.
As others already have noted... Everything that's worth to say has most probably already been said.
