Here are a few simple tips to try and extend your bottom time. Get yourself properly weighted, carrying extra weight (of the lead, not lard variety) or too little, will cause you to breathe more as you fight to stay level in the water. Ask your guide on the first day to help you do a proper weight check and remember every time you change location, or equipment, you may need to amend your weights.
Move as little as possible, have you ever noticed how lazy your guide is (underwater of course!)? By letting the currents drift you and being neutrally buoyant, you will be surprised how little effort you need to put into diving which will in turn use less air.
Kicking rigorously like you are riding a bicycle will be far more exhausting and less effective than long slow kicks, or even practice and develop the frog kicking technique for ultimate ease.
If you really want to achieve the streamlined, relaxed and properly weighted ideal you can take a Peak Performance Buoyancy course which will cover many of these points in detail and give you time to practice in a calm environment.