If you have a back-up guitar that you usually don't use for gigs, make this your regular practice guitar... but never change the strings. As the strings get older and dirtier, they also get tougher to play. By practicing your runs and scales on this guitar, you'll be able to play faster and more smoothly when you switch back to your main guitar for gigs or recording. It's like baseball players who put the weighted "donut" on their bats for making practice swings in the on-deck circle. Once they're in the batter's box (having left the donut behind), they're able to swing faster and with more power. Try this and you'll see what I mean.