Your lats, core and back muscles are very powerful, good rotation lets you use them to drive the stroke.
Bill has a nice long stroke with lot of body roll. Sweet.
Kicking drills on your side are excellent for developing body rotation. Perform them in your drill work or as part of your warm-up. More in our Swim Smooth DVD Boxset!
If you fight the water, don't try so hard and slow things down. Give yourself time to rotate. A Wetronome would be perfect to help you co-ordinate this.