I can't stress enough how important is is to stretch after a training session. As a runner I find that each time I go out for a run I return with sore muscles and this soreness means that I need to stretch to help my body recover.
I've found that using a foam roller from time to time also helps to alleviate soreness and tightness of muscles. I've also been fond of using a trigger point ball on my hip flexors and to help reduce tightness in this hard to reach area.
I have also found this stretching guide that Runners World has put together quite instructive .