1. Don't rocketspam, especially if you're forcing your team to move forward or backward to avoid YOUR ROCKETS.
2. Don't follow cloaked spies into enemy territory. A lone spy in an enemy base can take out sentries, medics, and other things, while you're ACTING AS A GIANT TARGET. SENTRIES WILL FIRE AT THE SPY. PYROS WILL RUN THROUGH THE SPY M1'ING. PEOPLE WILL TRY AND KILL YOU AND ACCIDENTALLY KILL THE SPY WHO WAS WEAKENING THEIR DEFENCES.
3. Don't get hit by rockets. Not only does it hurt a lot, but the splash damage affects everyone around you. If you're near a medic who people are falling back to, one rocket could take out your entire front line because they're injured and the rocket splashed off your chest.
4. If your best class is being played by your entire team, PLAY SOMETHING ELSE. It'll open up your options and you might find another class that you're good at. I only started spying because we had AN ENTIRE TEAM OF SCOUTS ON DIRTBOWL.
5. Stop pressing 'E' if you're not hurt.
Medics: heal EVERYONE, as much as possible. Disregard being credit to team or skill level; in the long run, it benefits you to heal as much as possible to fill up your charge meter. You're much more useful that way than spamming needles when you've got somebody far more capable by your side.
Of course, if you want to take the kill, I'm not stopping you. I'm just saying that you're also there to support the team, too.
If a Sniper tells you to get behind him, do it so he can take out threats from a distance.
Don't jump in front of friendly Rocketman when they are firing their Rockets. You may not receive any damage, but the Rocketman will.