If you do use D&D for character creation, apply the following basic rules:
Stats should average to 15. Nothing above 18 or below 3 prior to racial adjustments.
Use average hit points, max hit points for first hit die.
For example, 20d6 would be 6+3.5*19 = 72.5, rounded down to 72.
Multiclassing: keep track of fractions in both base attack and saving throws, only rounding fractions down at the end. However, the +2 save bonus in a good save for level one only applies once to each save.
For example, a character with 10 levels of rogue and 10 levels of bard would have a base attack of 15 (3/4*10 + 3/4*10), a base fortitude save of 6 (1/3*10 + 1/3*10), a base reflex save of 12 (1/2*10 + 1/2*10 + 2), and a base will save of 10 (1/3*10 + 1/2*10 + 2).
Magical Equipment: I do use the rules for stacking multiple effects on a single item (DMG p. 288 "adding new abilities"), with one minor change. Given an item with multiple enchantments, the most expensive of those enchantments is at the base price, while all other enchantments cost 50% more.
For example, a ring of invisibility and protection +3 would have a market value of 47,000 (20,000 for invisibility, plus 18,000*1.5 for protection +3). If this was later upgraded to invisibilty and protection +4, it would have a market value of 62,000 (20,000*1.5 for invisibility, plus 32,000 for protection +4); and purchasing the upgrade would cost 15,000 (62,000 - 47,000).
Feel free to fudge the rules a little bit if you need to in order to make the character work right. For example, you might decide that your ranger wouldn't have favored enemies or an animal companion, but would have somewhat more magical power than a normal ranger. Or perhaps your sorcerer really needs to have knowledge: history in class.
Also, make sure your character is capable of functioning on his or her own. Take the leadership feat and pick up a cohort if you have to. Don't stat a 20th level bard and then act surprised when you don't start off in a party that can do your actual fighting for you. Do pick up some cheap healing potions if you have no better way to heal yourself. And so on.