If you needed to lose weight you probably had lifestyle issues. In addition to losing the excess you have to fix the issues. Yoyo dieting results from people going back to their same bad habits after they diet. The major risk from something as extreme as a 1000cal/day diet is nutrition--at that rate you have to be really careful what you eat to make sure you are getting all the vitamins and nutrients that you need.
You also have to be careful that what you burn off is fat and not muscle, so you shouldn't do that sort of thing for long. Your body will burn fat and muscle equally and since muscle burns more calories than fat does you'll wind up with a lower BMR when you're done.
As painful as it is, you should exercise through a diet like that, especially lifting weights, and also cardio. That will ensure that you maintain muscle mass. You're plainly not going to gain any mass on a diet like that, but you really don't want to lose muscle, you want to burn fat.
I would not recommend 1000/day for long, I think you're better off around 1500 or 1800 per day, and again, exercising including strength training to ensure you don't lose muscle.
