usually, when the inevitable 'block' hits me, I go back to one of my previous renders and begin turning it into something totally new....removing objects, terrains, trees, adding other things, changing the POV, texture mats, etc.  Once you get going, creative juices begin to flow, once again.  When that doesn't work, then I know it's time to just comment on other's work for a while and enjoy the break. 

Personally, I often feel the need to turn-out something different regularly, like clock work.  That works for a while, then the inevitable 'block' hits.  Also, I find a good diversion for me is to try your hand at fractals using Apophysis.  There is a free download on the Web.