I'd suggest three things:
1. Quick Feet
2. Low Dribble
3. Maintaining a low sense of gravity
I think it's important to recognize what kind of dunker you are (one-foot, two-foot, or able to pull off both). From the latter portion of 10th grade until age 19, I used to be an exclusive two-foot dunker. It was a lot of fun to pull off in practices and in layup lines, but it never worked very well for me in games.
Typically, what'd happen is I'd pick someone's pocket, race into the open floor, then take way too much time trying to figure out how I was going to try to rise off of two-feet for the cram. 9 out of 10 times trying to dunk off two feet on a fast break would either allow the defense to catch up to me and take me out or my feet would set way too hard and quickly, making me slip or have a rough transition like you mentioned.
As a result, I spent a hell of a lot of time re-training myself to become a one-footed dunker again. It turns out it's ten times easier to bust off dunks when it involves running as fast as possible and just rising, as opposed to having to worry about setting both feet while running at full speed. So if two foot dunking on breakaways is really providing you trouble, I'd suggest trying to make the switch over to one-foot dunks.
However, if two-foot dunking is what you have your heart set on, in addition to the three things mentioned above, I'd also suggest staying light on your feet. The only time I really dunk off two-feet these days is if I've found myself well ahead of the pack and can take my time. In those cases, I'm sure to make myself dribble low to the ground, half-crouching, as this loads my spring. I'm then conscious of staying on my toes and not trying to set my feet as hard as I used to.
Basically, a lot of the same principles for one-foot dunking applies for two-foot dunking. Both involve getting low and placing a low dribble ahead of you. Some folks just seem to be able to pull this off quite naturally. Here's a very natural example (notice the dribble, center of gravity, and slight hop-one-two step prior to takeoff): http://www.youtube.com/watch?v=-v0DGY5aWog