In addition to the good advice above, the best advice I ever got was to concentrate on inhaling slowly and completely before exhaling, and to exhale completely and slowly on each breathing cycle. Also trim gear to minimize drag, adjust weight so you aren't keeping BC partially inflated to compensate for overweighting.
Also, minimize unnecessary motion. (I learned this the hard way
Don't kick into current, esp. on a drift dive unless you have to. I used to try to "go back" to a spot to see a turtle or something somebody pointed out after I drifted past a spot. I'd end up winded, and go thru a tank in no time. I learned on a drift dive to maintain buoyancy and keep as still as possible, letting the current move me along.
(The first couple dives of a season, I still use air fast. About the 3rd dive I'm finally relaxed and use lots less air. See Mike's post, above.) My first dive after certification, I breathed the tank down to 500 lbs in 12 minutes!
My wife is younger than I, fitter than I, and weighs 55 lbs less than I. She uses about 1/3 to 1/2 less air than I do.
When we do a deep dive (80-100 ft), and I know I'll be using more air because of depth, I'll rent a 100 cu ft tank for that dive while my wife uses an 80 cu ft. (That way I can usually stay down the full time for the given depth and not cut the dive short because I'm low on air.) Hope all this helps.
Have fun!