pred makes you retain like crazy; the first 2 times I was on it I managed to also pork on 30lbs in 5 weeks. The third time though I was bound and determined I would NOT gain weight. (I gave myself 10lbs leeway mentally due to fluid retention and the buffalo hump/moon face) I did Carb Addict's diet. I Only gained like 2lbs and it came right off.
The key to not gaining with steroids is watching carbs! Bigtime! The steroids make your insulin go crazy so if you don't go high protein low carb during this period you will be eating massive amounts of food; it makes youc rave like crazy.
I remember for my bedtime snack one night I had 6 hotdogs, fully dressed, in the bun. One another night I couldn't sleep (insomnia due to the pred) and ate 1.5lbs of smoked gouda with a steak knife sitting in front of the computer. I wish that was a typo. Anyhow, take care and watch those carbs!!!