Going low carb (REAL low carb) can be hard. IF you haven't checked it out, the "EAT MOAR FAT" thread that PaleoBird started a couple weeks back has lots of good info on ketosis:
I'm not saying you necessarily need to be in ketosis, but some of the same principles apply to a VLC diet. It can take a few weeks to become fully adapted to a much lower carb intake. Also, one of the key points made in the thread is that many people who try a ketotic diet eat too much protein to effectively be in ketosis. Dr. Peter Attia's blog is also a great resource cited in the above thread:
The Eating Academy | Peter Attia, M.D. The Eating Academy | Peter Attia, M.D.
I hope some of this helps.