I'm a huge fan of practice really slow -- uncomfortably slow. It not only trains you to understand the nuances of the space between notes, but also -- believe it or not -- helps to develop speed as well. It keeps you from locking into a standard comfortable tempo.
Another trick is to play groves and vocalize a "fifth limb" part. Basically, that means have certain notes in the grove (whether played or not) be vocalized. For example, while playing a rock grove, say the "e" of "e-and-a" out loud while counting the other 16th notes in your head (do this with all of them eventually). Then move to syncopated vocal parts, etc. I got that idea from the book "It's About Time" by Fred Dinkins.