I love the C-172RG - great plane. I agree with everything that madlandrover advised.
To save time and hassle in the air, learn the systems inside out before you fly, particularly:
1) Constant speed propellor - how does it work, how do you operate it, what happens if you lose oil pressure, what settings do you fly at. Try to read around this as well as there is a lot of useful information on the internet.
2) Landing gear - again, how does it work, what happens if you have an electrical failure and/or the pump fails, how does it operate under normal conditions (I have a personal rule that I always ask myself whether the gear is down on short final - no matter what I am flying).
3) Cowl flaps - again, how they work, what they are for and how/when they should be used.