Let me start by first saying, I kind of hate Alignment. Mainly I hate any restriction in roleplaying, which is why I have my own system, but occasionally Alignment is helpful, and players want it. So, I wanted to have some fun with it, make Alignment tracking more concrete, and add some more immediate consequences for good and bad roleplaying.
The way my system works is that each player begins the game with 2 Alignment Points (APs), one for each Alignment category (or 2 in the case of True Neutral). As the game progresses, the DM awards more APs for actions that exemplify each part of an Alignment. So while a Lawful character paying a fine won’t gain them an AP, helping track down a criminal would, and taking it upon himself to destroy an entire criminal syndicate might gain him 2 or 3 points. Each day a character gains at least 2 APs, they may add a +10 bonus to any Skill Check or Attack roll. They must use it within 24 hours, and must declare its use before they roll.
You may wish to reduce this bonus in your own campaign (even as low as +2 or +3), as I give out APs sparingly, and tend to ignore minor Alignment actions. The basic idea behind this bonus is that when a character is doing what they want to be doing, they are content, and that contentment shows itself as an extra boost in confidence.
Every time a character engages in actions against his or her Alignment, they will also gain APs. Whenever this occurs they receive -2 to all of their Skill Checks and Attack rolls for 24 hours, due to extreme depression and an existential crisis. Again, this can be lowered or ignored if you’re a more lenient DM than me, but I love to watch players squirm.
When opposing APs are equal to a PC’s chosen APs, or if they number at least 4 (whichever is lower), a Will Save DC 18 is made. The DC increases by 2 for every opposing AP gained above 4. Failure causes an Alignment shift. I also offer a +1 bonus to the Will Save for each AP in their chosen Alignment above their opposing AP total.
An Alignment shift carries all the standard penalties, but additionally they cannot use any Alignment bonuses, and suffer -1 to all rolls until their new Alignment is firmly established.