Every sport has a certain posture that aids in the enjoyment and sucess of that sport, whether it be wrestling, baseball, basketball, karate, or even playing the piano (sport?). Obviously correct posture (form, stance, position) is not held rigidly at all times, but it provides for a foundation upon which all skills related to that activity are built, and one of the toughest lessons to learn correctly.
Diving is no different. While you may be underwater, you are used to living on land. Now you must change your very perspective on how to approach tasks. In diving the word we will use for this stance/posture/position is "Trim". Proper trim in the water means being horizontal in the water column. It may be simpler to think of yourself as an airplane. Obviously an airplane does not sit vertically in the air (it would stall, descend at an unacceptable rate and most likely result in the death of its passengers ... not a good thing) Being in the water is similar (without the dramatics) in that when you are horizontal you increase the amount of drag on your body in the correct directions (up and down, making bouyancy control easier) and decreasing drag in the wrong direction (moving forward, backward etc.)
Consider a parachute - The reason it falls slowly is because it creates tremendous amounts of drag (by increasing it's surface area parallel to the ground). Being horizontal in the water accomplishes the same thing. You increase your drag when going up and down (which should always be slow and controlled) thus when you inhale and exhale the effects of your change in bouyancy are lessened. You can increase your drag vertically even more by keeping your fins parallel to the surface of the water.
Consider an arrow- It does not fly through the air sideways, rather it cuts through air with as little drag as possible at the tip (I know the drag of the feathers stabilize it and keeps it from tumbling, but stay with me on this idea...) As you swim through the water you want to reduce your drag as much as possible to reduce your air consumption and minimize your chance of overexhertion.
By now you are probably wondering how to attain this type of posture in the water. It's actually very simple to achieve, but first you need to get your weight placement determined. Here's how to do this...
1) Pocket style weight belt
2) 2 weight pockets (I like the ones from XS Scuba) or a BC with trim pockets (They will be little pockets on the back of your BC, sometimes hidden - many divers wonder what these little pockets are for, as they are impossible to reach while diving ... now you know)
3) Your Scuba kit that you normally dive in, including wetsuit, BCD, etc.
4) Proper amount of weight. Several should be in 2# blocks (Two 1# blocks are nice to have as well)
5) A helpful Buddy
1) Place the weight pockets on your upper tank cam band, one on each side of the tank but as close to your body as you can slide them.
2) Get neutral in the water and horizontal (I'd recommend doing this process in 20-40 fsw as bouyancy issues are less than nearer the surface, but be careful not to lose your bouyancy). You need to be completely horizontal, and keep your knees and head up (see later in this article) or this exercise will not work correctly.
3) Clasp your hands and cross your ankles. (in other words, NO movement is allowed)
4) observe what happens ... do you go head up (the usual first result) or feet up in the water? (we'll deal with rolling left or right in step 7)
5) Have your buddy move weight from your weight belt to the pockets you put on your cam band (several if you quickly went feet down). Obviously if you went head down, you will need to move weight in the other direction - one way of doing this is to move your tank lower on the BCD (the valve weighs a couple of pounds) or, if using a SS backplate, by using an Aluminum or Kydex plate instead.
6) Repeat steps 2-5 until you no longer go feet down or feet up, but stay horizontal with little or no effort.
7) Next issue, did you roll left or right during the exercise? You may need to move some weight from one side to the other (move weight from the left pocke to the right, or the other way around).
8) You may find your trim is not exactly horizontal when you relax (might go very slowly head down or up) because you are a few ounces heavy in one direction or another. This is easily solved if you have fins that are a little negative (like ScubaPro Jet fin style rubber fins) by bending your knees to move your feet closer, or further away from your body - this changes your center of gravity very slightly, allowing you to fine tune your trim while diving due to subtle changes (like an Aluminum tank getting lighter during the dive and ending up light at the tail end).
9) Dive lots - as you do, you will find your body adjusting to small changes on it's own. It just takes practice at this point, but you now have your weight placed where it will help you in this process instead of getting in the way.
Now that your weight placement is correct there are a couple things to remember. Probably the most difficult thing is keeping your knees up.
While you may be floating in the water, gravity still causes your knees to droop. After all, we don't use the muscles to keep our knees up while horizontal very often while walking upright. The first dozen dives or so you may end the dive with sore muscles as a result of this. Start by laying down on a flat hard surface (the kitchen floor for example) now lift your knees slightly off the floor and keep them up for 30 seconds. Do you feel the "burn" in your muscles being used to accomplish this? (Your gluteus maximus or "butt" muscles)
Don't worry, it won't be this difficult once you are in the water, but you now know which muscles it takes to keep your knees up. After an hour in the water however, your gluteus maximus will be getting quite sore and tired from holding your knees up, but it gets easier each time you dive as you build the muscles.
Why is this so important you may ask? If your knees "droop" they can effectively double the drag it takes to push you through the water, which means 4x as much energy to keep your speed the same (simple physics). Plus it causes your fin tips to point doward toward the bottom, stirring up more sediment (and ticking off your buddies)
Once you have proper trim ingrained into your muscle memory while diving you can change your attitude in the water at will, while keeping in trim and allowing you to have more fun as you "glide" through the water instead of having to push your way through it.
Secondly, new divers often find a backplate style BCD will help with the process of "trimming" out as it moves 6# of weight over the back and heavier fins give you a bit of weight you can move "on the fly" to adjust your trim mid-dive.
The best way to tell if you have gotten your trim squared away is to have a buddy take pictures of you (without you knowing). If no camera is available you can do a sand trim test. Slowly let yourself sink towards a sandy (or other safe) bottom. Your torso should touch the bottom first before your knees or chin.
The last thing to remember with proper trim is that where you look your body will tend to follow. So keep your head up. Some divers like to keep their head pinned against the valve on their tank as a reference point.