I have a few tactics I use when that happens.
1. Dig out an old favourite . Reginald Hill usually does it for me.
2. Change tack. If I've been reading a lot of darker stuff, I go for a cozy. If police procedurals are getting stale, I try for a PI
3. I never let myself read anything by the same author too close together.
4. Change genre
5. Read a real stinkeroo. From cover to cover. Yes, it's painful but it gives you a real appreciation for the good stuff. (if you want the name of a stinkerroo or two contact me privately and I'll give you the names of some real doozies.)
I've been lucky this past month. I've read 4 in a row that are probably going to be on my tops for 2007 list.
Hope that's helpful to you.