If you've ever done a good workout and just stopped and sat down, you know what lactic acid build up can feel like, and it's not pleasant. During an intense workout, your body can't produce enough ATP, and as a result, acidosis will occur and cause the discomfort. So how do we prevent this unpleasantness in the muscles? You need to cool down.
Performing cool down exercises and stretches will help your body and ensure maximum results. What exercises and/or workouts are good for cool down? Here's a short list that helped me out during my time in track during high school, and Basic Combat Training for the Army. Keep in mind, the less you do for a cool-down, the more chance of the build-up you'll get.
If you just get done with a race or a run, the best thing to do is actually to keep going. Once you're done with race or the run, start slowing down gradually, don't come to a stop right away. Run at a slight jog for maybe 100-200m, then walk for another 100m. If you're not good with distance, after your run, jog for about 20-30 seconds, then walk for about 30-40 seconds, and you should be good.
Overhead Arm Pull
I did a whole lot of this during Basic. You take your left hand, and put it over your head, placing it in the small of your back (or just below your neck). Take your right hand, grab your left elbow and slightly pull until you feel the stretch. Hold it there for 20-30 seconds. Once you're done with your left arm, repeat the process with the right arm. If you don't feel a slight burn, you're not doing it correctly.
Side Arm Pull
This is similar to the Overhead Arm Pull, except your pulling it to the side. Take your left arm and reach across your body to the right. Put your left arm on the inside of your right elbow, and bend your right arm at a 90 degree angle to hold it there, and just pull back until you feel the stretch. Hold it for 20-30 seconds. Once completed, switch arms
For the abs, interlock your fingers together and hold your hands upside down above your head, so that the palms are facing the sky. Lean back slightly until you feel a stretch in your abs. Hold it for about 20-30 seconds.
Again, interlock your fingers, but instead of above your head, place them straight out in front of you, palms facing out. Extend your arms until you feel a stretch in your chest and hold it for 20-30 seconds. Once complete, you can do an inverted Chest Stretch, in which you place your arms out like a T and move them to the back while standing still.