Pretty much what ren and rickblacker said is what I do when I get stuck in those ruts. Taking some time off helps in essentially "resetting" my mind. Ill pick up the guitar a week or two later and I just start feeling a good flow, It feels as if my imagination is refueled. Than after awhile I realize Im just repeating myself and get bored. Than I take a break again, etc etc.

Within those breaks from guitar I typically will concentrate on my singing. Once I get bored of the singing ill pick up the guitar again.

Also as ren said, once I see myself getting bored I try pointing myself in a different direction, such as instead of doing metal/shredding stuff ill try doing some classical guitar. Than I start doing the singer/solo guitar type stuff on acoustic.