Sinking hips during recovery in fly

Hi, I'm a butterfly beginner and currently having problem with my hips sinking too much when my arms start with the recovery. I posted some videos at my blog (http://blog.grkovic.com/?p=30) Hips sink so much that first downkick barely lifts them above them the water. Sometimes, they don't even come out. If anybody would have any suggestions, I would appreciate it a lot. Thanks. - Predrag.