The best way to stick with training I've come across is to use a logging heart rate monitor and the training load plugin for sportstracks to monitor fitness and fatigue via TRIMP. Sounds complicated, but it's really simple – you get a number for each workout saying how hard it was, a number which shows how fatigued you should be based on recent training, and another number showing how fit you are based on recent training.
http://gpsrunner.blogspot.com/2008/12/training-load-plug-in-for-sporttracks.html
Watching your fitness go up over time is super motivating, and you can easily get an idea of when you should rest, and can see your fitness drop when you don't train.
Cheapest way of getting a logging HRM is get a cheap non-encoded HRM from somewhere like decathlon or aldi, and the use the oregon scientific logger from Wiggle.