Sometimes I think I over-think these things.

In which I apply way too much logic to the Hulk. Again.

[small Avengers spoiler. spoiler. spoiler spoiler spoily spoiler. this is getting monotonous, but i suppose it's better than being beaten by a rampaging mob of comic geeks.]
Throughout the Avengers movie, everyone else keeps pestering Banner to tell them what he's been doing to keep the Hulk under control. A couple of them ask about yoga, seeing as they found him in India. Tony suggests a "big bag of weed". He never does dignify any of this with a response. The closest he comes is the line that everyone's quoting now, where Captain America tells him that now would be a really good time to get angry at the alien invaders, and Banner replies, "That's my secret. I'm always angry."

Nobody seems to know exactly what this line means. I'm surprise there aren't more bitter nerd-fights over it. Sounds awesome, though.

One detail of note is that Norton's Banner was absolutely obsessed with this heart rate monitor he wore, presumably working from the hypothesis that the transformation was triggered specifically by his heart rate. He does know that anger can cause his heart rate to climb into the danger zone (at one point, the monitor shows him hitting 200 bpm, which is not necessarily fatal, but given his age, is pretty squarely in Not Very Good For You territory, even without involving the Hulk), but he seems to think that anything that raises his heart rate beyond a certain level is a bad idea, including boning Liv Tyler in a cheap hotel room. It's so important to him that after he Hulks out and breaks the one he had, Betty brings him another along with his replacement clothes and the other miscellany he'll need to go on the run again.

He's no longer wearing it when Natasha picks him up. It looks like a regular watch. It's not a combo model either; the monitor beeped very pointedly when his heart rate climbed, but there's nothing of the sort on the soundtrack when the crew are arguing in the lab, when Banner hits the deck below and starts to change, or when he transforms willingly for the battle at the end. He also seems to have gained a specific awareness that it's anger -- or more broadly, upset or frustration -- that drives the change. If they're going roughly by the comics, and they seem to be, then it's a decent into fight-or-flight survival mode that provokes the Hulk, and calming down from that state is what brings Bruce back.

In that case, it's entirely possible that Tony is kind of right with his half-serious guess of pharmaceuticals. There are a number of different drug classes that are known to tamp down the fight-or-flight reaction, although many of them would be problematic for long-term use, because of addiction issues, because they interfere with thinking, or both. I really can't see weed being the drug of choice for someone as bright as Banner, plus it would make traveling difficult -- it's illegal in enough places that crossing borders with a supply would be problematic -- so Tony's not likely that right.

There is, however, one drug family that I know of which acts on stress hormones and pretty much nothing else. "Beta blockers" acquired the name specifically because they block the action of beta-adrenergic receptors, which are what mediate the fight-or-flight response. Stressful things don't directly make your heart beat faster, your palms sweat, your hands shake, etc. Instead, stress prompts your adrenal glands to secrete adrenaline, which enters your bloodstream and circulates around the body. Your assorted innards are equipped with receptors for the stuff; when an adrenaline molecule hits a receptor, a sort of knob on the outside falls into a hollow in the receptor, the receptor then snaps shut, and the stuff attached to that specific receptor kicks into overdrive. Beta blockers are beta-adrenergic antagonists, which is medico-talk for "thing that prevents the receptor from snapping shut". Thus, even if adrenaline is released into the bloodstream, the body doesn't respond.

There's an entire family of beta blockers, but one of the grand-daddy drugs in the class, propranolol, is on the WHO list of essential medications, and would be one of the easier things for him to get a hold of even when he's shuffling around in third-world countries. The official indications are for treatment of tachycardia and other cardiac arrhythmias and for migraine prophylaxis, but propranolol is also fairly widely used off-label in the US for treatment of panic disorder or situational anxiety like stage fright. It's not a controlled substance, so crossing borders with either a personal supply or a clinic's worth would be easy enough. There are also a couple of reasonably convenient antidotes to the stuff -- notably atropine and isoprenaline, of which the first is also on the WHO essential medications list for use in case of bradycardia, as a component of anaesthesia, and for a variety of ophthalmic work -- in case he needs it to wear off like right now. If Banner's got extensive enough medical training to work as a doctor, then he'd know all about it.