What do you do when your weight loss hits a plateau?

This is a huge problem for me, and it's what has been keeping me from my ideal body weight for years.
I've stuck on a steady regime of low carb diets and excersising, and I'm controlling my calories to match my total metabolic rate in a negative manner which is about 400 calories down (in order to burn more and not gain more).

So what do you do when you reach a stalemate?