You are the most important member of your team, bar none. Scouts are completely useless, relying on your generous nature to survive more than a few seconds in a firefight. Pyros, demos, heavies and soldiers require your help constantly, probably because they're only played by the intellectually challenged. Snipers and spies don't do anything anyway.
You are the person behind every cap and every steal. Never let yourself forget it. Oh, sure, Valve weighted the scoring system against you, but don't worry; deep in their heart, that heavy who charged the last point knows it's actually you that won it for the team.
Everyone is only shooting you because they like you so much. As a relatively meek class, it's clear that your partner's only job is to keep you alive. Their only job, mind; no capping, no taking the intel, no kicking ass.
So, every time you die, it is somebody else's failure. Make sure you remind them of it.
Any time you uber is a good time to uber. You're sure to get people complaining though. "Too soon", they'll say when their incompetence prevents them from destroying the nearest sentry gun. "Too late", they'll say when they run around a blind corner into half the other team and you decide not to follow.
When you're met with such criticism, you have a few options. The poor medic laughs it off. The mediocre medic just keeps on playing. Let me educate you as to the technique of the AWESOME medic.
Have a pen and pad sitting beside your keyboard. Any time anyone (yes, _anyone_) criticises you, mark their name down. Put it in nice, bold lettering, so that you can see it at a glance.
And the next time that player is calling for a medic, the next time that player is on thirty health and burning rapidly away to zero, the next time they're looking directly at you and want nothing more than to be healed, the next time their screams of "MEDIC!" fill your chat window: that time, turn your back on them. Find the nearest scout or sniper, and heal them instead.
Triage is harder than it looks. Say you have a few soldiers, burning, running toward you. The noob medic immediately starts evaluating their health and position, to see which he or she should heal first. WRONG! The correct tactic is to recall which player last said thank you, or credited you with a win. That's the person you should heal.
And don't get all soppy and try to save all of them. You can't (even if you can). That's life. The people you like get healed, and anyone who criticises you is toast.