Hey! I've been diving commercially for a couple of years. My philosophy (this might not be ideal for everyone, but it works for me), is to be very slightly negatively trimmed with my BCD/dry suit when maintaining depth, and then use my lungs to "make up" the difference. So when I breathe in a normal amount of air I am neutrally buoyant. I have a quite slow respiration rate when diving, so I "hold" my breath for a few seconds before exhaling normally and inhaling normally. I believe the "trick" to buoyancy control is to be relaxed and comfortable, and keeping your heart rate and respiration low. Do everything in "slow-motion",and try not to exert yourself too much. The more energy you use, the more oxygen you consume.