A couple of months ago I brought along a Bhatinov mask to the CAS meeting. I've started to use it to help with the sometimes tricky and always crucial task of focussing for photography.
You can download a mask specific to your equipment setup, print it, laminate it and away you go for about 50p!
The results of the mask are fantastic. It's incredibly easy to use and provides pin-sharp pictures.
The principle is that a diffraction pattern is created, with three (well six really) spikes. The central spike moves laterally between the two outer spikes to the left or the right as you move the focus. When you are in perfect focus the middle spike is centred between the other two as shown in the image below (taken last night).