I use a mirror to see behind, and look over my left shoulder to check the blind spot.

By looking over your right shoulder you won't see anything in your left blind spot.

IME other bicyclists are easy to miss when they're overtaking you when just checking your mirror (yeah, yeah I know all you guys NEVER get passed by other bikes

When looking over left shoulder I put a bit of pressure on right handlebar which keeps me in a straight line. Don't know if this is the "conventional method" but it works for me