Weaving in and out of heavy traffic is a sure sign of a careless rider.
When you have to change lanes or change position in a lane, make sure there’s no overtaking traffic. When it is safe, look, then signal and look again before changing lanes.
Turn your head and make a shoulder check. That’s the only sure way to see a vehicle behind you in the next lane, so it’s particularly important. There’s very little chance a driver in the next lane can react quickly enough to avoid you once you’ve started to move.
On a roadway with several lanes, check the far lanes as well as the one next to you. Another driver may be headed for the same space you are.