If you have the opportunity, learn to play the drums. The biggest improvement in my rhythm playing and understanding occurred when I started playing (with) the drums. I'm still not a good drummer, but at least I can accompany someone playing another instrument, or explain my drummer what I want him to play.
Think of something that seems difficult, nail it and gradually speed it up. I like playing bass drum on 1 and hi-hat on 3 (pedals only) in 4/4 rhythm and trying to add a 3/4 or 5/4 beat (sticks only) on top of that.
Another thing, learn to tap your foot while playing.