Bouyancy, that elusive, yet essential key that differentiates a mediocre diver from a good (or great) diver. As you learned in your open water course there are three statest of bouyancy: positive, negative and neutral. For the purpose of this article bouyancy means "Neutral".
When I teach, I tell my students the order they deal with situations are as follows:
1) Keep your breathing in check. (calm, collected, steady)
2) Maintain neutral bouyancy.
3) Then deal with the issue calmly and deliberately.
Your breathing pattern affects your bouyancy more than almost any other factor once your weighting is correct. When your depth remains constant, but your bouyancy changes look first at your breathing. Has it sped up, slowed down, become erratic or become shallow?
Bouyancy: Before you can easily work on your bouyancy make sure your weighting and trim are dealt with first.
If you do not have your weighting correct, and are fighting to maintain proper trim achieving bouyancy control will be much more difficult and take MUCH longer to learn.
For initially learning bouyancy control simply lay on the bottom in good trim (a good time to do this is while you are doing your weight check) inhale and exhale normally. If you do not rise off the bottom on an inhale add a little "puff" of air to your BCD (by little I mean "little" just bump the inflator button) continue breathing normally for 5 seconds, then add another "puff" of air until you begin to rise and fall off the bottom with each breath. As you practice you will learn how long to press the inflator button, but a common early mistake is to add large amounts and not wait for the change in bouyancy to occur before adding more. It takes several seconds for a change in lung volume or BCD volume to affect your bouyancy, this time "lag" is even greater if you are in good trim (this is to your advantage once you learn how much to add). The reason for this "lag" is due to your inertia. Once you start moving you must not only add air to correct for negative bouyancy, but you must also wait for it to slow your movement down. If you add so much air (or release) to abruptly change your movement you've now added (or released) too much.
You will most likely notice that you begin floating a few inches off the bottom during this exercise, without even trying. That is what you want to happen!
Next make sure you are familiar with all the exhaust dumps on your BCD. Most BCD's will have at least two. One is at the end of the corrugated hose (usually the button on the end of the inflator. Just remember "end" for exhaust and "inside" for inflate to remember which button does what.) The other exhaust valve will be on the bottom (if standing up) of your BCD, it may be on the left or right depending upon the manufacturer. Your BCD may have other exhaust valves as well (some BCD's have as many as five, but two are all you need) If you are not comfortable using the other exhaust valves, consider doing a couple of dives while only using the one which you don't normally use.
Once you are comfortable with the above exercise (hopefully this happened during your open water Scuba course) try doing a dive where you can reach down and drag your finger in the sand at any point during the dive. Imagine a bird skimming the surface of a lake that dips it's wing and brushes the surface of the water. Take into acount the fluctuation you will see due to your breathing pattern, but use this distance as a rule of thumb.
For the next step determine a depth you want to maintain during a portion of your dive and regardless of what the bottom topography may do swim at your predetermined depth. Eventually work up to doing this at a depth of 10 feet (or 5 if you really want a challenge)
All these assume you are in water clear enough to see the bottom during these exercises. If you cannot see the bottom watch some of the floating matter in the water column (it usually doesn't fluctuate much up or down) remember to check your depth gauge ocassionaly, but don't stare at it as you may find yourself having a harder time (glancing at it every 5-10 seconds should be more than often enough)
When you are comfortable with these exercises start practice going over objects (coral heads, rocks, etc) by changing your breathing pattern to adjust your depth. Approach a rock that is 3 ft high keeping your depth consistent, stop, adjust your breathing pattern so you are breathing deeper and have a longer inhale, watch as you slowly ascend (slowly being the key) when you arrive at the depth you want change your breathing pattern again to adjust for the new depth (instead of using your BCD) by breathing more shallow (be careful of CO2 build up in your lungs if you continue to breathe shallow over several minutes) once over the rock exhale deeply to begin a slow descent, and once you begin decending go back to your normal breathing pattern using a couple deeper inhalations to slow and stop your descent if necessary. Remember you're only moving about 3 ft up or down for this exercise. This is the same procedure to follow for ascending at the end of a dive with the added task of exhausting small amounts of air from your BCD while you ascend.
Once you are comfortable holding your depth during a dive while swimming along it's time to start practicing being still and holding depth. This is very useful for safety stops. Holding still (no hand sculling or finning now, that's cheating) makes you put several pieces together, incorporating breathing to keep your bouyancy under control and slight adjustments to your posture to keep your trim in check.
As you become more comfortable with your bouyancy it's time to start adding some task loading. Start small and increase it a little each dive until you no longer think about your bouyancy. A good place to start is with mask clearing while neutrally bouyant. Seems easy until the first time you try it. But the key is to keep a steady breathing pattern, this way no matter if your eyes are open or closed your bouyancy remains the same.
The steps I follow for a mask removal and replacement are:
1) Steady breathing pattern
2) Remove mask strap
3) Allow mask to slowly fill with water
4) Watch breathing pattern, keeping it steady
5) Remove mask from face
6) Watch breathing pattern
7) Place mask on face, strap behind head
8) Breathe normally (getting the idea?)
9) When it is time for a normal exhale, simply look up and use that normal exhalation to clear the mask. If for some reason the mask does not completely clear, simply wait for your next exhalation in the cycle ... (though this means your mask clearing technique needs some work, as you should be able to clear your mask 3-5 times easily on a single breath)
8) Continue breathing normally.
When you are comfortable with all your basic skills that you learned in your Open Water course try playing a game of tic-tac-toe while neutrally bouyant with your buddy (good safety stop game) - and yes, waterproof notebooks are made - just ask your local dive shop.
One last (though not unimportant) aspect of bouyancy is suit compression. Thicker suits will compress as you go deeper and you will lose the bouyancy they provided. To make it up you will add more air to your BCD (anticipate this as you descend, and be ready to vent in anticipation of expansion when you ascend) You could lose (and then regain on ascent) as much as 16 pounds of bouyancy (two gallons worth of air) just from suit compression if you are wearing a very thick suit. And when you replace this lost bouyancy with air in your BCD remember you are adding ALL the lost bouyancy to one location above (or around) your body. This may affect your trim as well.
At this point, it's just a matter of practice. Most divers who actively work on their bouyancy control will gain a modicum of sucess in about 25-50 dives. However you will find that even after hundreds of dives there will be room for improvement. This is probably the most difficult to master of the basic skills. And every time you add a new level of task loading you have to refocus on your bouyancy again.
Have fun and go DIVE!