There are a couple of myths - in separate categories - that I see promulgated in the fitness community and popular consciousness related to metabolism. I thought it worthwhile to take a moment to clear them up.
Without getting overly technical, what we mean by "metabolism" is the calories burned by your body at rest - otherwise called Basal Metabolic Rate or Resting Metabolic Rate. These things may not be exactly the same but they're close enough for our purposes.
Basically, even if you sit on the couch and watch TV all day, your body burns bunches of calories just keeping your heart and lungs going, your body temperature normal, and maintaining itself. This sounds like a great thing, right? Calories burned means they're not being stored as fat. So everybody and their mom wants a super high metabolism, which means your body is like a little furnace burning energy, and you can eat tons of food without getting chubby.
There are two general myths regarding metabolism:
1. Eating food X or doing Y will elevate your metabolic rate. This isn't always wrong, but it is usually wrong or overstated. Stimulants (caffeine, green tea extract, and so on) might elevate your metabolism but only a little bit and only for a little while (you get used to them quickly). Otherwise all coffee drinkers would be thin, and we can see that's not true. Eating multiple meals a day won't work either - you get an energy burning effect from food (called TEF), but it's proportional to meal size, so splitting your bigger meals into smaller, more frequent meals will do all of jack for your caloric burn.
Vigorous exercise will elevate your metabolism as your body repairs itself, but it's not an enormous increase. Would burning an extra 50 or 100 calories be nice? Sure, but that's what, half a Twinkie's worth of calories? It's not exactly a licence to eat junk food.
There are supplements that will significantly elevate your metabolic rate, but they tend to have serious side effects - thyroid hormone among them. I am far from an expert on the use of pharmaceutical substances to aid fat loss, but they're out there and your nearest pro bodybuilder is probably using them.
Here's the bigger myth:
2. Raising your metabolic rate is unequivocally good. This isn't so much something people say as what they imply. Marketing campaigns for various products will often tout how good they are for your "metabolism." Diets do the same thing. A particularly egregious exampe is the High Everything Diet (I won't provide a link because it's crap, but feel free to google it). The HED advocates say that by eating a certain way - lots of starch, protein, and fat but no juice or artificial sweeteners - you can elevate your metabolism and eat tons of food while losing bodyfat.
Here's my question: even if they're right (and I don't think they are), would that be a good thing? Do you want your cells churning away, burning tons of energy more than you need, sucking calories away from your system?
Think about a car engine. What happens when two identical engines ar run at different rates - one faster than the other? Outside of a pretty narrow range you can count on the higher-revving engine blowing up quicker than the other one. Trust me, Formula One engineers don't turn the revs up when they think their engine might be on its last legs. All those processes in the cell that burn energy - what else do they do? They result in some damage to the cell, even if it's a small amount, as they oxidize all those lovely fats and carbs to generate ATP. I'm sure there are few short term consequences to this, but over the course of a lifetime of wear and tear, do you want your cells working hard or gently when you're at rest?
If you want to be healthy and intact for a long time a slow metabolism might be advantageous. Calorie restriction, the one way we know of to increase a mammal's lifespan, certainly doesn't make the cells rev faster or at a higher temperature. It makes the animals run cooler and slower.
I certainly wouldn't argue that slowing your metabolism more is always better, either. Being cold and lethargic all the time, whether it extends life or not, is not a fun way to live.
Take home message: if you're lean, or in the area of lean, and relatively healthy (have a decent amount of energy, etc.) then speeding up your "metabolism" may not be a good thing. Be wary of diets, exercise plans, and supplements that claim to do so - they probably don't work, and if they do work they might be limiting your longevity. Conversely, if someone criticizes your healthful lifestyle on the grounds that it might slow your metabolism, just smile, nod, and walk away. You can have the best revenge - outliving your enemies.