hi
To increase your time under water try learning how to relax. Im the worst deep diver in the world but what has helped me to get a small amount of depth is to close my eyes on descent this makes me loose track of depth without focusing on the bottom. I find that when staring at the bottom all the way down makes me anxious to get there quicker so I start kicking really quickly.
Also keep a very relaxed grip on the gun handle, I almost dont hold my gun at all. I find that if im nervous or cold I hold the handle really hard which is not a good thing cos you should be totally relaxed.
Another good trick that Aquiles taught me is, if your using a float line try coiling some of it up in one hand then as you start descending let it go this will make a friction free descent cos you arnt fighting the drag of your floatline pulling along the surface as you take your line to the depths.
On the bottom try not to make to much forced effort as this will severly hamper down time, its better anyway to stay still and even toss some sand up over your head some species like this and will come close. I find that when you are seriously trying to stay down for long try not to have your head looking to much of an up angle if you know what I mean its puts strain on my neck muscles and makes you feel out of air. A better way is to keep your neck straight and relaxed if possible.
On the ascent I like to give a few hard dolphin kicks with my bi fins. You may like to try using the dolphin kick instead of the scissor kick, I found that it boosts you to the surface a lot quicker and easier then scissor kicking.
When preparing for the next dive I like to wait till my heart rate gets back down to 60bpm then take around 5 deep breaths and go again this I usually wait on the surface for a lot longer then others and dont dive until I see a good fish or find some promising bottom terrain. But if the vis is really crap you may have to dive up and down all day, some guys here stay down for 2mins then surface for only 1:15 and repeat that is nearly impossible for most people.
Im only a 20m diver so take my advice as such, hopefully some of the DEEP guys can step in and lend some advice
cheers