I agree with what everyone is saying in general about relaxing, doing things slowly, your environment and water type, ect. What you really need to do is go back to the basics of what you were taught (hopefully) for all that. Sure, we were all excited on our first several dives and had to learn to relax - everyone has been the newby coming up before the half hour mark while the "half man-half fish" people stayed another 45. Like so many have said, with time and being more relaxed and comfortable with your skills, your bottom time will improve. I do have a few suggestions my wife and I have used:
I tend to use up my air faster than the rest of my family (me, wife, 2 teenagers), so we have made it a practice for whoever is partnered up to share air for a few minutes here and there DURING the dive. This helps those of us that use air faster to have more bottom time, plus it's a great skill to practice with your buddy. It does not affect our diving in any way as we have full reg octupuses on every set of gear and just continue our dive while breathing off of one tank for a few hundred pounds before going back to our own air. It does not halt the dive in any way as we just continue along together instead of waiting until the safety stops to get the extra air. Also, all 4 of us are totally comfortable with air sharing, so if there is ever a "situation" during a dive requiring the need for this, that is one less thing for any of us to stress over.
Another thing we noticed pretty soon after we began diving is that we were using quite a bit of air on buoyancy - many others have mentioned this already. We have basically cut our weight in half over time which has helped (with experience). But if we still find ourselves needing to adjust our bouyancy often during the dive, we try to use the manuel inflator over the automatic inflator button. It works fine - you just need to practice it alittle to breathe in from your reg and out into your inflator hose. Again, it's another basic skill that is not a stressor for us since we use it regularly to conserve our air and gain bouyancy when having trouble.
I hope some of these suggestions have helped. I am amazed at how my bottom time ( and skills in general) have improved as I have gathered experience. Divelog #10 in Coz and divelog #90 in Coz were SSSSSOOOOOO different!! Good luck!