Play games other than hold'em. It forces you to think about theory & strategy in an original way (since there isn't much reading material on other games). And I find that remembering how to think strategically for other games causes me to do so more often when I'm playing hold'em.
It gets you back in the habit of going "okay, on this flop, how am I going to make my opponents make a mistake?"
Sweat sessions are always helpful too.
I agree with this too, a while back when I was playing non stop hold'em, things weren't necessarily running bad but hold'em was losing the fun, it was like I was going through a phase and the phase was ending. I picked up omaha and the fun of poker came back, I got the chip back on my shoulder to prove that I could play a different game. I played omaha for a good period of time at least a couple of weeks, when I went back to Hold'em everything was fresh, I had a different outlook on the game, and I was back in hold'em with a chip on my shoulder.
like everyone else said a break is always good too though, sometimes thats all thats really needed.