One of the body roll drills my coach had me use was putting a kickboard between my thighs above my knees like a fin, and feeling it "slap" the water. It should slap at even intervals on the right and the left as you swim. It also helps me become really aware of tight hip flexors - I have a new respect for how your hips move apart when you swim.

I have also practiced taking 3 strokes then rolling to my back, taking 3 strokes then rolling to my stomach, then on the way back doing it the opposite way (start with the other arm).