dave, if you want to do well without a lot of effort, just play heavy.
Whenever you spin up the minigun, always jump first. Whenever you go around corners, jump and spin.
Be aware of sniper lines. Avoid them whenever possible. Try to keep to relatively confined spaces. The more people who can shoot at you, the worse off you'll be.
Right-click with the sandvich equipped to drop it. This gives you ammo and everyone else health. Use it to give your medic health.
When nothing is really going on, shoot behind you every so often.