Cooling down is a key component to finishing your workout. Whatever you do, don't skip the cool down. It's really important that you get your body back to its resting state, especially if you did anything to elevate your heart rate. You don't want to just come to a dead stop. You want to gradually bring your heart rate down. For example, if you just came out of a run, then you want to slow down your run and maybe do some marching in place until the heart rate starts to come back down to resting levels. That's also going to help keep blood from pooling in the lower body as well in the lower extremities.
Bring the heart rate down and then do some stretches. Think about whatever exercise it was that you did that day and you want to do something to strengthen or, if you did a lot of strengthening you want to do something to now elongate muscle. For example, if you did a lot of mountain climbers, a lot of high knee runs, a lot of squats, then you really want to open up your hip flexor.
Our hip flexors tend to be really tight anyways. Just from sitting, the hip flexor gets tight. For example, you could come into a split stance squat position, and then to get the hip flexor, what you want to do is tuck your hips under. That's going to be really what gets that nice stretch into the hip joint, into the quadriceps even a little bit. If you wanted to increase the stretch, you could take a side bend. Now you're getting into the obliques and all the way down into the hips.
Again, the key is to get into the lunge and tuck your hips. We call this posteriorly tilting your pelvis. Just think of your pelvis like if it was a bowl and you wanted the contents of whatever was in the bowl to spill back. That's the action that you're doing.
You could take it to the ground and get a little bit more. Then if you wanted to open up and do a really deep stretch for your thigh muscle, your quad muscle, if you did a lot of legs that day, you could come down, grab a hold of the ankle joint, either with the opposite hand or even with the same side. Just really get into that. It opens up the hip, making sure your knees stays over your ankle as you do that. That's also really important. Your heel should be down. Your knee should be over the ankle.
If you did a lot of upper body, open up the chest. If you did a lot of push-ups and things to make the chest tight, you want to now set the shoulder back into a good postural position. You could open and close the arms, dynamically move through the stretch, or you could have a friend hold your elbows gently and just pull back. Again, just elongating whatever muscle it was that you spent all that time in the workout contracting. Bring yourself back to optimal length, and bringing yourself back to resting levels. It will also help aid in your recovery, meaning you won't be as sore the next day. You'll be ready for your next workout.