In my opinion, too few pilots know what their exact fuel-burn is. There really is no point anybody saying "a 152 burns xx litres per hour", because it depends on how you fly it.
Crucially:
What power setting do you use?
Do you lean the engine?
If so, how do you lean it?
Every time you fly, you should measure (using a dipstick) how much fuel is in the tanks before your flight, and how much remains afterwards. This is the only way of knowing what fuel burn you get.
Once you've done this for a little while, then you can start having real confidence in your fuel calculations. Too many pilots just fill to the brim every time, thinking that this gives them maximum safety. It doesn't if they're operating off a shorter runway on a hot day - it actually diminishes their safety margins. And sooner or later you'll find yourself needing to depart an airfield that has no fuel. If you've never put your fuel calculations to the test then how can you know whether your departure is going to be safe? To take only enough fuel for your journey plus sensible diversion requires that you know for certain what your own fuel-burn is.
If you measure your own personal fuel-burn then you'll find it's incredibly consistent (assuming you always use the same power setting and leaning technique). You should be able to predict to the nearest gallon how much fuel is on board at the end of every flight.
The POH figures are the best starting point (even my 1959 172 burns fairly close to book figures if you fly using book technique), but ultimately you must ignore everyone else's figures and use your own.
Of course, all of this is made much easier if you always fly the same aircraft. But if you rent, it's going to be a little harder - but still not impossible.