The main way I get inspired is by looking at what other people have done. I'm not only looking at the ingenuity and beauty of designs, but also the techniques used. I see a really great grunge poster, or a great color scheme, and think, I can do that but tweak it this way or use it for this other idea I have...
Below are some great sites I use to see wonderful works done in Photoshop and get myself inspired. Some of them are tutorial sites, others are not.
Deviant Art (Type a topic and go)
These are just a few to get you started.
Have some great sites that you go to for inspiration? Post them in a comment below!