Everyone hates the guy spamming 'MEDIC!' repeatedly because he took 25 damage and doesn't feel safe. On the other hand, equally annoying to the doctor is a team full of Real Manly Men who never call medic. If I don't know where you are, I can't heal you. The fabled middle ground is when a team only calls medic when they need it, where they need it, when there's a reasonable expectation that they'll get it. In return, every time you call medic I'll do my damnest to get to you and pump you up.
What does this mean in game?
DO call medic if:
...you're in the red.
...you're skirmishing on the front lines without a medic. Even if you're not hurt too badly call medic and we can go from 'skirmishing' to 'laying waste'.
...you're around the corner from me or otherwise nearby but not directly visible.
...you want a buff - especially fellow medics (ESPECIALLY after you get more than 2/3rds ubercharge), scouts, suicide pyros, and stickyjumping demos.
...you're near the front without a medic.
...you're pulling back in the face of an enemy push.
...you're on fire and I can't see you.
...I died, you survived and now I'm respawning. If you let me know where you are we can pick up where we left off.
...you're a combat class (soldier, heavy, demo, pyro) and you KNOW you'll be in combat in the next few seconds.
DON'T call medic if:
...I'm already healing you.
...you called in the last 10 seconds. (the icon is still above your head so in the meantime STFU)
...you're on fire and I can see you. Flaming people are obviously first on the list so if I'm ignoring you its for good reason.
...we're in a big firefight (like pushing the 2nd CP of the last stage of dustbowl). Everyone's taking damage, so I'll do my best.
...you're already running back to a health pack.
...you've got no reasonable chance of living. Damnit, I'm a doctor, not a miracle worker.
A few other tips that will make everyone's lives easier:
-As you're running up to me calling medic with between 50-100% health, bring out your secondary/melee, shoot, tap the voice chat button or do something to show me you're not a spy. I expect every last {TF2F} and [TF2F] to do this - I know you're all good enough to be up to the task.
-If you're a pyro coming back for healing, spam your flame around me while you hang out. You watch my back, I'll beam the bullets out of yours.
-If you're calling medic in a big group, try and pull out of the group a bit to make it easier for me to click on you. Even just jumping in place helps. Don't run through me or stand right on top of me, it makes you harder to spot.
-If you're <25% and I start healing you, don't immediately rush back into heavy combat. Every second I wasted healing your dumb ass is a second I could've spent healing someone less useless. If you wanna be Owen Wilson, get a new doctor.
-Other medics: If I call medic at the start of the match during setup it's because I've already buffed everyone else up to 150% but you're all ignoring me. Buffing the medics during setup is very important, particularly the medics who are about to rush out uber.
-If you're really hurt and I'm not getting to you, stay out of combat for 8 or 10 seconds so that when I do get around to you you'll heal faster.
-If you're not in an obvious spot, say where you are on voicechat. "Above you on the shed," or "in their sewers," or whatever.
Even a, "behind you, Doc!" will help in the fog of war. [Thanks FlopEJoe]