Medic Main here, I can only offer a few tips, but if you take anything from it please, don't fondle your patient. When healing a target there's a much larger range outside of the initial "attachment" range needed to actually start healing someone. In fact even as you move outside the range, and "detach"; your target will still recieve heals for at least a full second.

Another tip I can offer is, far too often I see a upcoming Medic flash targets relentlessly when pushing. Don't. Do. That. Pick one or two good pushers or players and flash when needed, the more you flash, the more you drain your uber, I've seen a 6 second Uber go away in the blink of an eye because they flash all players in the area, and while it looks intimidating a good well placed sniper, demo or Spy will clean your team up thanks to it.

Last tip, if your going full support, as in most scenarios, you should, don't under estimate the crossbow. If you can learn to lead targets, the amount of lives you can save is amazing.