The most important thing in these situations is to be positioned to take advantage of them. If you want to be bumped, don't book the last flight that gets to your final destination on time.

Also, book on airlines with lots of service between your travel points, this gives you more options for rebooking on later flights.

Take advantage of the heat. Hot air is less dense than cold air. Planes get less lift in
less dense air, so they must be lighter, thus leave some people on the ground.
So, in the summer, book mid afternoon flights, the hottest time of the day. The hotter the air, the more empty seats they need.