Some of my best work has been done at like 3am when I was frustrated and took a late night walk to clear my head. The best thing you can do while doing a layout is occasionally get up, push in your keyboard tray, and turn off the monitor. (save your work first, in case Felix decides to play on your keyboard in your absense).
The brain doesn't work well when subjected to the same thing for a long period of time, and the breaks will help make the work seem to go faster. Doing something quiet, unrelated to what you need to do will help because you get a fresh outlook on the project and ideas will start flowing again.
Planning is also important. I like to sketch my ideas onto a drawing pad before starting, that way I have a clear idea of what I want to do. Graph paper works well for layouts too, but sometimes they're too limiting...
Hope this helps, cheers