To get the most out of exercise, try stretching all major muscle groups at least 2-3 times each week, which totals 10-20 minutes of stretching muscles of the upper back, neck and shoulders, lower back, chest, quadriceps, hamstrings, and calves. It is also a good idea to stretch the muscles you use in exercise on a daily basis - for example if you are a walker, pay close attention to your lower back, hamstrings, quadriceps, and calves.
Stretch when your muscles are warm - the best time being right after your workout. If you are especially tight or are planning a very vigourous workout and need to stretch beforehand, make sure to do a gentle warm-up first. Gentle forms of yoga or t'ai chi are wonderful places to begin. By working on muscle relaxation and tension reduction simultaneously, you get a super-powered flexibility workout. Or try water - water is a terrific medium that allows greater confidence and freedom of movement without the risk of injury. Use flotation tools such as "noodles" to make your stretching more effective.