Depending on how long you have before you start your CPL you could pop into the school that you intend to train at and ask one of the instructors to sit down with you for an hour and talk you through how they teach the nav ex's. Most good instructors will have some sort of rules of thumb that they use eg: x miles per minute as a g/s, your thumb represents 10nm on a chart, drift correction etc etc. Some might have you using the 1 in 60 rule. Others might have little laminated "cheat cards" with g/s, drift, time, which can go onto your kneeboard.
Also have a look over the Navigation book of the Trevor Thom manual for a bit of the theory.
The best bit of kit I ever brought was one of these:
http://www.transair.co.uk/product4.a...Product_ID=978
Great for the VOR fixes that you will be asked to do as well as for diversions.