Part of having good buoyancy and trim is having the right amount of weight. You should float at eye level with an empty BCD and holding a normal breath. When you exhale, you should slowly sink. See Your Skills as a Diver II - Weight Check and Proper Weighting. What should happen when you exhale?
1) You should float at eye level
2) You should sink slowly
3) You should rise to the surface
4) You should stay at the same level