I'm going to say this up front, and hit the point again later. If you don't have the knowledge to mess with the trigger mechanism on a firearm, DON'T! I'm not telling you to work on one, and I take no responsibility for what you may do to any firearm. This is simply a rundown of some things I've learned that I'm passing along. As Kim says, no warranty and mileage may vary.
Trigger work is probably the most sensitive thing you can do with a firearm. If you mess up a magazine, it can be replaced, same for the stock. If you're crowning a barrel & mess up, you can generally trim a bit off the length and do it over. But if you mess up with a trigger, it may not show up until sometime later; say, when it fires the piece because it was jarred. VERY bad juju, guys. So if you don't know what you are going to do, and how to do it properly, then DON'T DO IT.
Assuming the trigger works properly, the usual problems are that it is too heavy(i.e., you have to press harder than you should for it to release or 'break'), or it's rough, or both. First step is to take the piece down and clean it thoroughly and oil it. If it's an old gun that hasn't been taken down in a long time, that may take care of the problem, and you'll have to do this in any case to see what needs to be done; you may find that it's too bad to mess with, or that something needs to be replaced. A lot of times treatment with a really good lubricant will do all you need. If that doesn't take care of it, then things get tricky and call for great care, because one or more engaging surfaces or bearing surfaces may need to be polished.
Warning again: if you don't know EXACTLY what you're trying to do and HOW to do it, put the stone down and step away from the workbench. No matter how fine the stone or compound you use may be, it doesn't change the fact that stoning or polishing the surface is REMOVING metal; if you take off too much, or cut the angle incorrectly, or change the angle of a surface, you're screwed. Metal taken off cannot be put back(yeah, I know it CAN be- IF you're a welder and have the RIGHT stuff to build it up the RIGHT amount and cut & polish it to the ORIGINAL shape & size and then HEAT-TREAT it), so don't take anything off you don't absolutely have to. Or you'll be finding a replacement part at best; at worst you may wind up with a hole somewhere you didn't want one.
Mauser-style triggers are one of the easiest because there's only three surfaces that may actually need anything, and they're flat surfaces. A good stone can be held flat against them and worked back & forth to polish them, just the minimum, please. Some you shouldn't mess with at all. Handguns can be tricky for a variety of reasons. A semi-auto taken to too light a pull weight can in effect go full-auto on you because the jarring of the action working causes the sear to slip and let the hammer fall, NOT a good thing. Revolvers generally don't have that problem, but you can still make the pull too damn light for safety. And if you take too much off other parts, you've got a different problem. Say a Smith & Wesson double-action revolver. Polishing the pin the hammer pivots on, the sideplates where it bears, the rebound slide and the frame surfaces it bears on are standard ways to smooth out the action, but if you take off too much, it makes things sloppy and you've made the action worse; and there's no easy way to fix this. And remember, if the piece is new, trying to do this stuff yourself will probably void the warranty, and you won't sneak this by them; the 'smiths at the company know exactly what they're looking at, and will know that you did something you shouldn't have. Besides which, if it's a new one, it needs to be used a while to break it in before you even think of touching it. Once a new guns parts wear in with each other, that may take care of everything, so have patience.
Say you've taken an old rifle down, and found that the engagement surface where the trigger holds the cocking piece is rough, and you decide that you can take care of this. First, make sure that if you cut the surface down enough to fix the problem, it won't create another problem by making the surface too small/too low, etc. You're sure? Then implant this in your cranium: NEVER, NEVER TRY TO DO IT FREEHAND! Set up some kind of jig or guide so that the angle is correct and stays that way. Companies like Brownells sell adjustable jigs for just this purpose. I've used a drill press vise, a roller and a level to set up a jig for a simple piece. However you do it, use something; trying to do this freehand is asking for trouble. It's nearly impossible to keep an angle exact, and if you do it won't be flat, it'll be slightly rounded at best; more trouble.
And you can't use that $1 pocket sharpening stone you picked up at the flea market, either. You need something small enough for easy control, with truly flat surfaces and known abrasive level. A coarse stone will cut too fast and leave a rough surface. A fine stone, no coarser than 240 grit, cuts slower and leaves a better finish. A 400-grit is better, and if the surface just needs polishing you want much finer than that. Again, Brownells is a good place to look. These stones ain't cheap, but they can leave a surface like a mirror, and if you take care of them they'll last forever.
And yet another warning. Some triggers/sears/etc. are not made of a medium or high-carbon steel that's been heat-treated; they're made of soft mild steel that is case hardened or surface hardened. That means that there is a thin skin, say a couple of thousandths of an inch that's hardened steel; if you cut through that the hard surface is gone, and it will wear very quickly. Which means you have to buy a replacement.
I cannot stress this enough: find out EXACTLY what you're doing BEFORE you start. Find a good book that covers the work and study it. Brownells collected tips sent in to them by gunsmiths and published them in a book called 'Gunsmith Kinks'. There are four books in the series now, with a lot of good information. They'd be a good place to start.
One more thing; this stuff CANNOT be rushed. If you don't have a stable work surface, and the tools and knowledge, and plenty of time to work on it, then don't. From knifemaking and forging I've had many cuts and burns, and carry a couple of scars, that are the result of trying to rush something. Getting in a hurry is a good way to have to start all over, probably on a replacement part.
I'll finish this with my disclaimer, again: I have no control of what you decide to do, and take no responsibility for what you do. I write this to pass on some knowledge of just how tricky and precise this kind of work is, NOT to tell you to try it. If you decide to work on your own firearms, then a: understand WHAT you're going to do, b: understand HOW you're going to do it, c: HAVE the right tools, and d: take your time.