If you are neutrally bouyant, your own breathing can bring you up or down slightly and this is the ideal condition for most diving. However when you want to surface, you add more air to your BC. If you want to go deeper, you let air out of your BC.
Here's some tips from YukinDiver.com:
Some General Tips on Bouyancy
Bouyancy control in colder waters, when wearing full, thick wet suits or dry suits, is much more demanding than when diving in the tropics, wearing just a T-shirt or a thin shorty wet suit with no hood, mitts, etc. When wearing thicker neoprene suits, there is much greater compression of the suit material when descending in the water than when wearing very thin suits. As you found out, we must wear much more weight on our belts when diving in colder waters to compensate for the thick wet suits - as we descend, the air bubbles within the neoprene compress, causing us to become heavier in the water. This will become very apparent even at a shallow depth - even less than 10 feet. As the neoprene compresses, we become heavier and must, therefore, keep adding air to our B.C.D.'s to maintain neutral bouyancy. Since, as we descend, we keep adding air to our B.C.D.'s, it then follows that, as we ascend, we are going to have to let this air out before it takes over control and we begin an uncontrolled ascent. It is important to maintain vigilance over your bouyancy and be pro-active by letting excess air out of your B.C.D. when ascending before you loose control. This becomes much more important when diving in a dry suit because you, now, have two air chambers to control - your suit and your B.C.D. As with any skill, practice will make your bouyancy control automatic - you will do it without even thinking about it - usually.
How do you maintain bouyancy control - I seldom see you touch your inflator? The answer to this question is very simple - use your lungs! The average person's lungs can affect their bouyancy by 4 - 6 pounds. This is a lot of change - especially when you are neutrally bouyant to begin with! When I dive, I adjust the air in my B.C.D. to achieve basic neutral bouyancy, then I adjust the mid-point of my breathing to fine-tune my bouyancy. If I find I become a bit heavy, I'll begin breathing a little deeper to compensate. When this becomes noticeable to me, I'll reach over and add a touch more air to my B.C.D. or to my dry suit, as required, to take me back to neutral bouyancy. This way, if I want to look closely at something on the bottom, I simply lift my feet up and let excess air out of my lungs until I descend a little, then I breathe in deeper to rise back away from the bottom again, never using my hands to propel me. Perecting this technique will give you much more confidence in your ability to control yourself in this water world.
