The key thought is to not pay people extra to do their job well.
If they're not motivated to do a good job without you needing to entice them, they're not right for your organization. You can't buy professionalism.
It's been my experience that doing a group event is much better than individual cash bonuses. (lunch out to celebrate, a movie, golfing, arcade afternoon.. or a office event does more to raise morale and bring everyone together -- you have a laugh, relax and things are good.)
Folks just want to be appreciated, in thought, action, and not just by throwing money at them (it is a nice gesture too though)..
Bonuses are fraught with pros and cons
Pros:
(+) People who do more should get to share in their hard work. Question is, how do you fairly measure and assess it across the board for tangibles and intangibles?
Cons:
(-) People who don't pull their weight might qualify for a bonus
(-) People might consider they are doing you a favor and expect even more bonuses for other things
(-) productive contributors will feel upset their contribution was hurt or went to others that didn't pull their weight
(-) people may wonder if someone got more or less of a bonus, or if someone got it at all