Before committing to a manoeuvre make a last check over the shoulder into the blind spot to make sure nothing unexpected is happening!
If you are turning, use it to check the blind spot on the side to which you intend to turn.
Getting into the habit of making a quick check over your shoulder before pulling off, changing direction, or changing lane might just save your life one day!
IMPORTANT: Use your judgement about when to make a shoulder check! In urban riding these checks are normally essential, but during high speed overtaking when you are certain what is happening behind it's generally safer to keep your eyes on whats happening on the road ahead.