I've tried just about every diet plan under the sun and come to the conclusion that very low carb is the only sure-fire way of maintaining at the lower end of my 'happy' weight range (54-55kg), I'm a HFLC fan so keep carbs to around 20-30 g per day. When you've done this for a month or so and counted calories you will get a feel for what and how much you can eat each day. Weigh daily or weekly to keep check.
I've been maintaining in the 55-57kg range since 2005.