Weight gain is a relatively common side effect of Prednisone. This is usually due to increased appetite and fluid retention. Good news is, this usually comes right off once the Prednisone is stopped.
The best way to combat this is just to really watch what you eat! I've been on prednisone of varying doses (up to 80 mg + 80 mg kenalog at the same time, all the way down to 2.5 mg at one point) for over a year now, and I haven't gained a single pound. I'm also tough on myself when it comes to what I eat - I eat relatively clean, and when I get a craving for something (Nutella is on my brain a LOT recently....) i have a little bit and when my body says "more!!!!!", I remind myself it's just the prednisone talking (;