Make a mark of some sort to lead you back on some Medkits that you didn't have room for before. That way, you can find them again if you run out. This is useful in Interval 11 with the armies of ghosts coming after you. Just make sure that you are able to backtrack to get the Medkits you left behind.
Use your Medkits at the correct time; don't use Medkits immediately when you have about 70 to 80 health remaining. Try using a Medkit when your health drops below 30. One Medkit can heal up to about 30 to 45 points. If your health happens to drop below 40 and you are in a 1v1 or 2v1 fight, do not bother using a Medkit unless absolutely necessary. Just run to cover while using your slow-mo abilities. Your health will regenerate slowly back to 40 if it drops below that, however you must be out of the firefight to regenerate your health.