The biggest mistake I have seen players make (including myself when I was younger) when trying to dunk is stutter step before they take their last two steps. When you stutter step before jumping you are minimizing your maximum potential. By stutter stepping you are decreasing your speed thus minimizing your vertical leap. 

You don't stutter step before a layup do you? Well don't stutter step when you are going for a dunk. Pretend you are going for a layup except all you are doing is jumping higher and grabbing the rim. 

Negatives for stutter stepping:
  • Less speed
  • Less vertical 
  • Your leg muscles will tire out faster   
and you will look silly 

I bet you will gain 2-4 inches on your vertical jump by taking away your stutter step.