How to not look like an idiot in Team Fortress 2: the Heavy Weapons Guy

When playing as him:

-Find a medic buddy

The HW Guy is pretty powerful on his own, but he's also tragically, irritatingly mortal. I've seen HW newbies run out, guns-a-blazing, under the severely mistaken impression that their 200+ points of HP will keep them alive for longer than ten seconds in the face of a Soldier, Demoman, Pyro, or other HW Guy.

Finding a Medic buddy (usually by hitting the E key repeatedly until someone on your team gets so annoyed they switch to the Medic class just to shut you the hell up) will triple the length of your average life, and will force your enemies to reconsider their strategies in dealing with you.

I'd link to the Penny Arcade comic about this, but you've probably already read it. So, screw that.

-Get the minigun ready before heading into battle

Holding the secondary fire key -- which should be bound to right click, unless you're insane or something -- will rev up your minigun, requiring but a single click of the left mouse button to fire. Attempting to fire by holding the left click requires a good second and a half of waiting, as the HW Guy lowers his minigun, sets it a-rotating, and then fires. If you find yourself in a position where you're at least 50% sure an enemy is around a corner or in a room you're about to enter, there's no reason not to be immediately ready to start firing.

Conversely, though, keeping your minigun in this position slows your walking speed from "turtle" to "turtle dipped in maple syrup with two of its legs cut off." It also makes a hell of a lot of noise, generally ruining any and all attempts to ambush the bad guys. Only rev your minigun when you're pretty sure there's some killing to be done; no more, no less. 

-FREQUENTLY check your backside

Spies derive a downright insane amount of pleasure from stabbing Heavy Weapons Guys. They consider it one of the most helpful things they can do, aside from sapping enemy sentry guns. Even ignoring all the damage the HW Guy can do and therefore how much danger he presents to an opposing team, the HW Guy is slow-moving, generally played by people who don't want to do a lot of thinking, and has a huge goddamned back that simply aches to have a knife inserted into it.

That in mind, always Always ALWAYS check your backside as a Heavy, whenever possible. Hell, you don't even have to stop firing; if a Spy is cloaked somewhere behind you, a few seconds of spray from the minigun should kill him. If you see anyone approaching you from behind who isn't a medic who is already healing you (Spies love to disguise themselves as Medics and approach Heavies from the back -- the only way to tell the real deal from the fake one is that Medics will heal you immediately from a distance, whilst fake Medics will insist on getting extremely close before stab-stabbing you), hit them with a few seconds of minigun pain. It doesn't really require that much effort or time, and the rewards of frequently doing this are incalculable. Nothing pisses off Spies more than an intelligent Heavy Weapons guy.

-Learn to love the Ubercharge

Many Heavy Weapons Guys -- hell, many Team Fortress 2 players in general -- tend to play with reckless abandon. "Hey, if I die, I'll respawn in a few seconds," they think. Usually this attitude results in Scouts running headfirst into flag rooms or Spies excitedly attempting to backstab every enemy they lay eyes on, but Heavy Weapons guys usually exhibit this ignorance of death by not respecting their Medic's ubercharge.

The Ubercharge takes a few minutes of sustained healing to fully charge up, and, for most Heavy Weapons Guys, this is too long. Since they are, after all, Heavy Weapons Guys, they already consider themselves kind of invincible -- the addition of a Medic only multiplies this feeling. Therefore, most HW Guys don't really take the Ubercharge into account when formulating strategies; of course, they like it when they find a Medic who is 95% charged and can activate it whenever necessary, but they won't usually wait outside a flag room as their Medi-buddy's ubercharge increases from 70% to 100%.

Well, you should.

Apart from the really obvious benefits of goddamned invulnerability, there's not much more that needs to be said about the usefulness of the Ubercharge. Waiting a mere one or two minutes in exchange for ten straight seconds of total invincibility is a small price to pay, especially as a class who can effortlessly deal with enemy attackers during those said one or two minutes. Be patient; it'll pay off.

-Use teleports

Heavies, as mentioned earlier, are ESS-ELL-OH-DOUBLEYOU. Take teleports as frequently as possible (so long as you have a general idea of where they exit, anyway).

-Keep an eye on your ammo

The feeling of invulnerability every HW Guy experiences is often inextricably linked to a feeling of carrying unlimited ammo. After all: 200 is a pretty big number when viewed out of context, and most Heavies are too busy worrying about the state of their medic or pwning their next beeyotch to keep an eye on ammo conservation. 

You'll find, however, that a little bit of conservation (when it comes to the Heavy, at least) goes a long way. The ammo crates sprinkled throughout each level usually contain enough minigun ammo to keep the Heavy supplied up until the next ammo crate, and the miniguns of fallen enemy heavies will usually do an even better job of helping you along.

Regardless of how you get ammo, the only important thing is to know that you need ammo. Heavies can provide a great deal of useful covering fire, and given their massive killing capabilities, they tend to eat up ammo rather quickly. Mind your reserves and don't be afraid to run back to recharge your minigun. Yeah, the Heavy has a shotgun and his fists, but they're really no substitute for a fully-loaded chaingun.

When playing against him:

As a Spy, engage your cloak really early when walking behind him; this gives you the opportunity to memorize his movement patterns and learn when he turns around to check his back (if at all), and therefore when to strike. If you're disguised as an enemy combatant (I'd suggest Scout), always run past the Heavy first so he will mentally cross you off his "To Kill" list, then either double back around and stab him, or wait for your team to start shooting in your general direction, thus giving you an excuse to act scared and backpedal. People usually don't fire at teammates who run backwards toward them -- exploit this fact and gank the Heavy just when he thought he was safe.

As a Sniper, you should have a pretty easy time headshotting the Heavy, so long as he stays in plain view (which he really shouldn't).

Pyros and Demomen are great for setting Heavy ambushes; the Pyro's flamethrower will usually ignite both Heavy and Medic, confusing the two and allowing other allies to attack them en masse. The Demomen's sticky mines, if he places enough of them hidden sufficiently, will be powerful enough when simultaneously detonated to kill any Heavy, regardless of how many Medics are healing him at any given time.

If a Heavy is hanging out with a Medic, go after the latter first. Obviously. If you absolutely can't go after the Medic because he's intelligently standing behind a doorway or something, have your teammates concentrate all their fire on the Heavy. Make sure to strike simultaneously; if a grenade hits, then a few seconds later a sniper hits him in the chest, then a few seconds later a Soldier bullseyes him with a rocket, the Heavy will probably either have been healed up in between attacks and cancelled out the damage done to him, or he'll have just enough health left to retreat with the Medic and fully recharge himself. Don't give him that chance.

Always sneak up on a Heavy's back, whenver possible; they're somewhat slow to turn, so a few extra seconds of surprise damage to them can make all the difference in the world (especially if you're a HW Guy as well).