I learnt a great deal from my GUE Fundies about my buoyancy control and trim, and I reckons it boils down to 2 important factors: a good instructor and video feedback. While the first one is a given, video feedback is such an invaluable tool with teach. It's like with golf, swinging in front of the mirror. It provides you with instant info on whether you're doing it correctly, and it helps build the necessary internal feedback. If you have a kind friend with a camera, you could have your friend help video you and then review the clip to see if you're doing the right stuff.
Good luck!
Good luck!