Alex,
I agree with all the answers so far. You should do a bit of navigation, spend some time in the circuit, and practice your general handling. If possible, spend a bit of time with an instructor every now and then to check you're not picking up bad habits - doing a checkout on a new aircraft, tail-wheel differences or an IMC rating are good ways of spending time with an instructor.
For the navigation, do not use GPS. Plan your route, plan headings and times, and then be sure to put the chart away and trust your headings and times, making corrections at a checkpoint (e.g. half way) if they're not working. Practice diversions by taking someone with and asking them to give you a diversion, or asking them to write a diversion and seal it in an envelope which you only open in-flight. Do not be afraid of airspace - make sure you get plenty of practice crossing control zones, MATZs, etc.
For the circuits, make sure you have the correct approach speed on every approach. Aim for a point slightly before where you want to touch down. Doubleu-anker is absolutely right to point out that x-wind technique is important, and is very often not taught properly on PPL courses.
For the general handling, don't be sloppy - if you're practicing stalls, not only should you do a HASELL check every time, but make sure you maintain your height as you do the HASELLL check, for example.
Mental arithmetic will definitely help, but Piltdown Man may be going a bit OTT suggesting you can do SIN and COS in your head. The clock code gives a perfectly good approximation. However, you will need to know your 6-times table and your 3-times table to use the clock code (and those two come in handy for other things as well).
Have a read of the CAA's
Standards Document 3, Notes for the Guidance of Applicatants taking the CPL Skills Test - this will describe what's involved in the CPL, including guideline tolerances (but really there should be no tolerance at all - if something isn't 100% right, then put it right, smoothly, as soon as you notice it).
Finally - and most importantly - have fun! This is the most enjoyable part of your training - use it to go to interesting places, fly interesting aircraft - whatever floats your boat. You want to be able to look back on this time and smile!
FFF
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