After neglecting my Takamine acoustic guitar for a few years, the frets became tarnished and dull. Many guitarists would recommend rubbing steel wool against the frets to polish them up, but that's a fairly barbaric way to treat an axe. Not only will it booger up a nicely finished fretboard, but you'll get a million microscopic scratches from the filthy steel wool dust.
A better way is to use a Dremel rotary tool with some polishing compound, as recommended by this web site: http://www.projectguitar.com/tut/fret.htm. I already own a Dremel, so I bought the Dremel Polishing kit
and some of the least "sticky" blue masking tape that I could find. The polishing process couldn't be simpler:
The results are excellent, and the shiny frets on my guitar play totally smooth.
Note about tape: My guitar has an ebony fretboard, so the tape didn't leave any residue. However, the back of the neck is varnished, and some tape residue did remain. A simple Dan Erlewine trick removed it easily: a little saliva on a cloth rubbed the stickum right out.