Alignment is really just a tag on your character that helps define how certain spells, character class abilities, and magic items react and interact.
That said, you get your alignment from how you play and define your character. If you have players saying "My guy does this because he's (x-alignment)" he has got it backwards. He should do what is consistent with his character or players wishes. And then assess later "yeah, I guess my guy is kinda chaotic..but basicly good.." or whatever. There's a fuzzy kind of fusion in there where you are defining your character through your own actions and you may even have an ideal of "lawful good" or "neutral" or whatever in mind, but what your alignment is is fairly subjective.
My character once pushed a helpless captive kobold into a pit one time after milking him for info. Was I being evil? Actually, the character was chaotic, and an expert on monster cultures. He suspected betrayal and trickery from the kobold and knew nobody else in the party would do the job. On the other hand, I've rescued goblin slaves and set them free when I knew they wanted nothing to do with us.
When alignment was used to dock people experience points, kick people out of classes, and as a pretext for complaining about how people roleplay (or lack the ability to roleplay) its no fun. It just seems counterproductive.
However, the way it works now is basicly a personality tag, and that tag can be used like any other stat (hit points or BAB or whatever) to do some interesting game effects.