420 Training Videos and e-book
The International 420 Association have produced an excellent 420 training video "420 to the MAX" which some sailors may already know. They have also recently added a 420 training "e-book" aimed at both coaches and sailors - this has lots of exercises to improve your 420 sailing, with links to videos explaining the exercises.
Both are available from the International 420 Association website (the videos are also on youtube).
420 Tuning Guides :
The following guides may be useful for tuning your 420.
420 Measurement and Documentation Guide