Suspension training is an excellent way to strengthen the upper body, even for people with serious muscle. One of the best forms of exercise you can do while suspended is pushing and pressing, especially with push up variations. Push-ups can be narrow or wide, incline, decline or completely vertical. For an extra tough challenge try doing suspended flyes.
With different angles you work different muscles. For example, when your feet are up in the air at an extreme angle, the deltoids get blasted. Working the triceps on the suspension trainer can be a real challenge as well. Warm up with standing tricep presses with the elbows locked in place and in line with the shoulders. Then lengthen the straps on the trainer a little bit more on each set. If this still isn't enough, start raising your feet on the rungs of a ladder. The incline is sure to provide the resistance needed!
For some great ideas on push-up/pressing progressions, have a look at the videos below: