Concentrate on whats coming ahead of you not whats behind you. One thing I learned from mtn biking is that your wheel wants to go where you're looking. If you're looking at the road edge then your front tire will naturally begin to migrate that direction.
Even if i have a shoulder i usually ride close to, or just to the left of the white line. There's usually crud on the shoulder but it also gives me someplace to move into if I need it.