Now, I only play medic when I absolutely have to. There's some medics out there that annoy me. So here's some tips from a me how to be a good medic to a sniper.

• Buff the sniper whenever possible. All it takes is for you to click me for 3 seconds when you pass by me and then I'll have an easier time taking on snipers who also happen to be after you.
• I'm second priority to heal. Your first priority is to heal the people who can deal damage when we're under attack. I will just be more careful not to take damage during the time you are occupied (even if I said "Medic!" and you're doing something important, chances are I just noticed and will help repel the threat before you can heal me).
• If I am the only one in the frontlines, do heal me and don't fallback, if it comes down to it I can defend myself. There has been several times where I have stopped a CP cap or grabbed the intel with the help of a medic when nobody else was around. However my counters tend to be your counters as well (scouts, spies), but with your help 95% of the time I can kill them.
• If you are healing me and we're both the only people around, be ready to fallback when I think we're about to get overwhelmed. You can tell when we're falling back when I'm generally running away from danger. It's not hard.
• Uber: Worst target for uber. Ubers are used primarily for the classes that can deal large amounts of damage or splash damage. I can only deal damage when I hit a really, really small target.
• Kritz: I'm an bad target for krtiz, but I'll likely get 1-3 kills if you give it to me. You're better off giving it to someone who has a lot of health and can deal long range/explosive damage. My sniper rifle won't be effected by kritz, and I'll die if I use my machete with kritz, however the SMG does about 24 damage per crit, which is alright. The only target which is worse to use the kritz on is Pyro, who has to be in danger to use the kritz.
• Don't latch onto me. Thanks for the buff, but you see that combat class over there? Heal him instead while I hang out behind you both and cripple enemies with headshots while you finish them off.
• Check behind you. If I'm there covering you and your healmate, and for whatever reason I'm dead, that's probably because there's danger like a scout/spy/pyro coming behind you, or a sniper who just took me out and is going after you next.
• Be a rear view mirror (even though I usually check behind me). Healing is fortunately really easy, so all you have to focus on is dodging fire and checking for threats. Get in the habit of checking behind you every 10 seconds for suspicious teammates and warn your healmate. In the heat of battle your healmate can't look behind, but you can.

If anyone has other things to add please add them from the perspective of your class of choice.