My Brazilian Jiu-Jitsu teacher (Andre 'Dedeco' Almeida) showed me this awesome Side Control drill when I was a new White Belt. It is one of the best drills I've seen for training Side Control/Side Mount and really emphasizes the necessity for each position. I use this drill to this day as warm-ups in my classes, and pass it on to benefit my students.

The drill connects various positions by number which is a big help. We need each of these in order to control our opponent on the ground when we have side control/side mount.

The drill works not only a solid position, but also the transitions from one to the next. You can increase the complexity of the drill by having your training partner call out the number and you go to that position. Once you have the drill down, randomize those numbers to make it harder. 

Note: Position 6 is technically not Side Control, it is known as North/South, but it makes an excellent addition to the drill, and something we often need when trying to maintain side control and someone is getting out.

You can train this with a partner for best results, or with a throwing dummy, punching bag, or body pillow. Enjoy the drill!