One thing to remember is that you probably don't need to let ALL the air out of your BC to descend. If you are properly weighted, you should be negative by the amount of gas in your tank, which is plenty to get down. If you are finning yourself up (any fin movement in a vertical position pushes you up) or holding a big breath, you may find yourself letting too much gas out of the BC, and when you let that breath out, or stop kicking, you'll sink fast.
Another thing that happens is, if you are wearing thick neoprene, it will begin to compress as you descend, and your descent rate will increase. Your first means of defense is to inhale, and then to puff a small amount of air into the BC to slow you. With practice, it is quite possible to slip under the water, get immediately horizontal, and stop 2 or 3 feet underwater, and then totally control the descent.