I wish I could tell you how to get yourself in the mood. That's something that I have to figure out for myself as well. What I can tell you, though, is not to try to force it. If you're not feeling it at the moment, then let it go. If you try to make it happen when you're not feeling it, you may find yourself playing with sort of don't care mindset. At least that is what I find, anyway. My attitude will be like it would be great if I did well, but if I don't care if I get knocked because I'm not really into it anyway, right up until I make an I don't care play and get knocked out. Then I get upset with myself because I don't really believe in playing bad. I don't care if it's a freeroll or a 100K Guaranteed. I believe that if you play smaller tournaments with like you don't care, you will develop bad habits that will carry over to the bigger tournaments and creep into your game when you need to play your best.
Like I said, if you're not feeling it, just don't play. One of the best examples I can give is playing the 100K GTD on Sundays. Like a lot of players, I tend to want to avoid paying the full buy-in if I can. In the past, I have won tickets through satellites, but don't actually use them until just before the event starts and I'm sure that I'm going to play in it. I once found myself with three tickets to the Sunday 100K. One week I didn't feel like I had gotten sufficient sleep the night before to be able to stay focused and play my best. The next week I felt myself starting to get a headache and knew that staring at a computer screen was only going to make it worse, and decided it wasn't worth it. In the end, a month passed from the time that I won the first ticket until I finally used it. The point is that if I'm not up to playing my best, whether it be due to ailment, fatigue or mindset, I simply won't play.