In high school, our cross country coach taught us a pretty thorough stretching routine. Since then, I really haven't deviated from it when it comes to running.
After yesterdays speedwork, my legs were stiff. I stretched and headed out for a quick 1.5 miles around the lake (I'm leaving early for a battery of allergy test YUK!). After the first lap (.75 miles) my legs were so stiff I stopped and stretched some more. My first thought was "I'll push through it" but I decided against that because I'm being extra cautious when it comes to any kind of injury right now. So, I stopped and did some leg stretches, mostly focused on my calves and shins. The upper legs were fine. After about five minutes of stretching, I took off again and finished the second lap fine.
Thinking about this now, I feel I should research the art that is a runner's stretch. I'm sure schools of thought have changed in the nearly 15 years since I learned this routine.