Wiping your strings down when you are done playing is important. My hands sweat a lot and my sweat will turn strings black after only a few hours of playing. What I do now that helps me keep my strings feeling good and lasting longer is I wipe them down the a cloth dampened with Dr. Ducks Axe Wax. It gets the gunk off and leaves the strings a little slick. It also keeps the strings from corroding.

Doing this I went from having to change my strings every 4 weeks or less to every 6 weeks to 2 months depending on how much I am playing.

Are you putting your guitars in a dusty environment? Do you play them everyday? I have all of mine out and I only notice dust build up on a guitar I may not have played for a few weeks.