All Class Tip:

* While running by yourself or to a destination always have your melee weapon out swinging to catch any unsuspecting Spies who happen across your path. Most effective when rounding corners, doorways or narrow hallways.
Don't Bother: Pyro, Heavy, Spy
* Spectating is a great way to learn new tactics. If a player is dominating a match spectate and see what he's doing
* Turn on Quick-switch-Weapons, the added time can be the difference between life-and-death
* When you're Ubered/Fighting and run out of ammo(specifically Soldier,Demo) don't be afraid to switch to your Melee or Secondary Weapon. You can kill a lot faster than if you shoot-reload-repeat
* Never run in a straight line always keep your movements erratic to avoid being an easy target for Snipers or Backstabbing Spies
* Remember Sentry's always attack the closest person so if an uber rushes in to take out a Sentry you can help in the attack unharmed as long as you stay behind the uber
* When you see an Uber do NOT waste ammo attacking them(see too many doing this), instead try to flank behind the Medic then attack as soon as it wears off. This doesn't pertain to the Demo's Stickies and Pyro Airblast which can knock them around negating its effectiveness
* BaggyGirdle Added: Even if you don't have a mic, use the text to communicate/strategize with your team (for example: Spy/Sentry locations, etc.)
* BaggyGirdle Added:
Use the voice commands(default "z, x, c" keys) to quickly communicate information like: "Spy!", "Incoming", "Need a Sentry/Tele/Dispenser here" or just to say "Thanks" after a Medic heals you

common sense

lol, its still helpful I suppose.

I'll quote something from ubercharged when it regarded to the post on Thirty-Six Rules of Fighting: Part 1 of 6

"The key principle of this guide is that it won’t tell you many things that you “shouldn’t” already know; what you’ll learn is everything that you should know but isn’t ever put into practice, along with things that you probably didn’t know, but yet play a major role in-game."

I already added a link to that site... perhaps just follow the rss and don't copy it all in here