May I suggest that you switch to a surface supplied air system... No, not really, I’m just kidding, of course.
It looks like everyone else has hit the important points: Keep aerobically fit, make sure that you are properly weighted and streamlined as you dive, learn to relax, breathing exercises, rebreathers are certainly an option. All of these things should help you.
I would add dive often. Even if you are in great shape, you may tend to consume a bit more gas if you have been out of the water for a while. I know that if I go more than a few weeks without diving, my consumption rate goes up for the first few dives.
Also, don’t be too concerned if you are not using your breathing gas at the same rate as your buddies. The truth is that everyone is a bit different. Even very experienced divers, who dive together regularly, will often come up different on their gas usage.
Good Luck & Safe Diving
Ron sends