A good cool-down will help your body recover from exercise, prevent cramp, aches and pains, tendinitis and fatigue, and prepare you for your next run!
The first good habit to get into is not just stopping and collapsing in a triumphant heap at the end of your run. you need to cool down gradually: go from a run into a slow jog, relaxing your shoulders and letting your arms hang loose, then slow into a walk. A gradual wind-down should last from 5-10 minutes, depending on the intensity of your run.
Then find a quiet spot and cover up so you don't get cold. Do some gentle but targeted stretching (legs, bum, back and arms), breathing deeply throughout. For example, to stretch your quads bend your right leg out behind you, keep your knees together, take your foot in both hands, bring your heel to touch your right buttock, stretch, then change legs. Whatever you do, don't force yourself into a stretch that isn't comfortable!
Help your body rid itself of the toxins it builds up during exercise by drinking lots of water (between 1-2 liters over a period of several hours after your run). Mineral-rich water is best.
Sleep is one of the keys to any healthy lifestyle, and its crucial to your recovery, so make sure you get your 8 hours of blissful shut-eye. Sport also helps you get to sleep easier, so no excuses!