Stretch 'heated' muscles —Stretching a cold muscle can strain and irritate the tissue. Warm up first. Walk before you jog, jog before you run, etc. It's most beneficial to stretch after you exercise, when the muscle is heated by blood flow and is more accommodating of a stretch.
Always warm up before stretching; stretching a cold muscle invites injury. Walk, ride a bike or do some calisthenics for 5-10 minutes before you stretch.

Stretching Basics
-Stretch after a warm up, before your workout, between sets and afterward for best results.
-Use static, not ballistic, stretching.
-Remember to hold your stretch for at least 15 seconds and as long as 30 seconds.
-Perform each stretch 2-3 times for best results.