People have thrown out most of my ideas, but I'm going to repeat them in one post.
First off, when you put on your dry suit, squat down and pull the neck seal open a bit and exhaust as much air as you can. You can repeat the squat (without pulling the neck seal open) once you are in the water (if you are shore diving). This gets a lot of the air out of the suit before you try your descent.
When you go to descend, hold your inflator hose up and release air from the BC until you begin to sink. Then take a big, deep breath. Most people will tell you to exhale as you exhaust the BC, but if you do, then just about the time your head goes underwater, you'll need to inhale again, and that will pop you back up. Instead, take a deep breath just as you begin to sink. Then, as your head goes underwater, sharply EXHALE and hold the exhalation for a few seconds. That will continue your momentum underwater. By the time you inhale again, you'll be far enough underwater to be able to do so without corking again.
A lot of the time, people are finning as they try to descend, often because they're tending to go over on their backs. If you bend your knees and cross your fins, you won't push yourself back up, and in addition, the water hitting your fins (which are now more or less horizontal, and presenting a big "sail" surface to the water) will tend to tip you forward into a horizontal diving posture -- all without pushing you back up.
The great thing is that you recognize that this is a technique problem, and not one to solve by packing on more weight. You WILL get past it -- I did, and when I met my mentor, the only way I could descend was to go down flat on my back until I whomped into the bottom. It only took a couple of dives using the tips he gave me (listed above) before I could descend in much better order.
