Relax your elbows/upper body. Let the bike choose its own line.
When you look over your left shoulder, bend your right elbow. When you look over your right shoulder, bend your left elbow.
Try to not look over your shoulders. Instead, look back under your armpits or between your legs so you're seeing what's behind you upside-down. You really only care about someone's wheel if it's close enough to you to see it this way.
When you're riding alone, and it's not raining, try to ride on the white lines on the roadways without coming off.
Do more close-quarters, fast-paced group riding.
Oh, and did I mention to relax your elbows & upper body?