You have my full sympathy while taking prednisone. Listen to the various advice given and see what helps you. But be prepared for the fact it's going to be tough while you're on high doses. I find the side effects are more manageable once you drop to 5mg/day.
Try to be strict on your diet and don't let any weight gain or moon face make you think you've lost the battle and get tempted to think I may as well eat what I want as I'm getting fat even on healthy diet. If you stick to healthy eating you'll lose weight a lot quicker as you reduce steroids as it will be water retention, rather than fat you're storing.