One of the best ways to improve your guitar playing is to put the guitar down and play something else! Most people find that once they've become reasonably proficient on guitar, learning another plucked string instrument is fairly easy. Learning a bit of five-string banjo can't help but improve your fingerpicking on guitar. Playing a bit of mandolin will probably improve your flatpicking. And if you're only playing in standard EBGDAE tuning, get some info on alternate tunings and experiment around with them.

A lot of great stuff can sneak into your guitar playing through the side door if you're willing to let it.