Alot of people will tell you that this something that should only be done by a qualified gunsmith. But, I've "improved" several on my guns, quite easily. the key is to not get carried away with a file or Dremel, which could result in a dangerous situation. Generally, all I do is use a diamond stone (or very fine file) to square up contact areas and then polish the mating surfaces (and occasionally, depending on the design, lighten the springs by shorteneing them a bit). The tricky part is figuring out exactly what parts to square/polish or which spring to change/shorten. This, of course varies alot form gun to gun, depending on design.
You may not get a "match grade trigger" doing this, but I've improved the triggers ona many of my guns quite a bit.