Clean the guitar when you change your strings.
You can get aroung the bridge,pickups and rub the nasty grime that builds up on the fretboard(sound like a tile-x commercial)You can't really effectivley clean with strings on the guitar,but its a good idea to take some cleaner designed for strings and wipe them down once a week(depending on how much you play)Ive heard that the strings last longer,but I havnt noticed.It will make them sound bright again though,like there new.Go to any music shop and they will have cleaning supplys.You could order them online too from musiciansfriend,music123,zzounds e.c.t.
Ives tried the GHS fast fret stick,but it left cotton all over the strings and made the worst sqeaking sound when applying it.Theres 3 basic cleaners.Fretboard cleaner/conditioner(lemon oil)string cleaner and body cleaner.Ive tried alot of body cleaners,but they all leave a thick gunk and swirl marks.Just take a guitar cloth(or soft cotton underwear)and rub it to a nice shine.

And andy82...does dried armpit sweat add character to a guitar?If so then my main guitar is Micky Mouse!