The stretching I do in the evening (or any other time I take a longer break) is basically the same as most runners, aerobics and martial artists use... heres a link to a basic programme for legs, arms and chest:
http://www.myexerciseplan.com/assessment/stretch.pdf
Obviously your legs are most in need of stretching after a days ride, although to take it a step further, neck, back and side stretches can be added. It won't take long... 10-15 minutes. Often I just stretch legs.

Don't force it, don't get too ambitious... relax and enjoy the pain it will be rewarding and slowly but surely you will become suppler and more flexible and be able to ride further and faster...

Some people have mentioned that beer they drink after riding... Apart from being a poison, alcohol IS a muscle relaxant hihi, cheers!.. you can actually combine the two activities, stretching and a (usually warm 'cos it's been in your bar bag for a couple of hours) beer...

Oh yeah if you do stretch before riding or when your muscles are cold, take it a more gentle and slow... personally I only stretch when warm which can be defined by AT LEAST 12 minutes of gentle aerobic exercising (bicycle riding for example).
Sometimes when you're really knackered and all your leg muscles are cramping and tensed up it can be easy to overstretch and cause muscle damage. Wait until they're more relaxed (or I suppose you could drink that beer first)...