If you want to help keep your strings sounding like the day you bought 'em for as long as possible... one thing will help above all else...
WASH YOUR HANDS!
Seriously! This "helpful hint" was revealed to me "the hard way" during a soundcheck about 20 years ago, when I innocently asked the lead singer to thump on my bass while I walked out front to hear what it sounded like on the PA. It sounded great for about a minute... and then suddenly (and quickly) started sounding muddier and muddier, till the bass just sounded like it was strung with rubber bungee cords. When I took the bass back from the singer, I noticed a weird orange residue on the strings and the fingerboard. Before I could ask the singer what the hell the orange stuff was, I noticed him walking happily away with his jumbo sized bag of Nacho Cheese Doritos.

The worst part of this horror story? No new strings for the gig that night, cause it was a holiday and the music stores were closed!
So again... and I can't stress this enough... for the longest-lasting strings...
WASH YOUR HANDS! 