A couple of days ago I was riding home in heavy traffic and was moving over across about 4 lanes towards the exit ramp. I have a 25 foot truck in the lane to my right and I start slowing down to move in behind him as soon as he passes. (Keep in mind, there are lots of cars, so it's not like I can get behind him at leasure, I have to squeeze it almost right away.) As he passes and I start moving into the space immediately behind him I realize that he's pulling a small trailer behind him. Because I looked to make sure no one was trying to pull in from the right side of that lane, I had more than enough time to avoid any close calls with the trailer.

In short, don't assume that
1. the space you're about to merge into is empty just because your mirrors say it is, and always look to make sure.
2. another vehicle will not be merging into that space at the same time, merge slowly and look to make sure no one's there.

This should be religion while you're riding