You should clean up all the empty cans or bawls bottles on your desk. A virtually empty desk gives you a sense of well being, zen if you will, and motivation to actually do something productive. Check if your computer is making too much noise. Your ears are so accustomed to the noise that that more often than not, you will realize how loud it is when you think about it consciously. That is the sad part, the humming noise is affecting us subconsciously without us realizing it. This is one of the reason that I am thinking about getting a Mac Mini. After supposedly spending $200 of "quiet mods" for my PC, I still ended up with an unacceptable noise level.
Turn off the damn computer:
Oh yes, you heard it right. Sometimes it helps to get back to the offline world. I dont know about you guys but for me, I prefer to design the structure and anatomy of my application on pen and large piece of paper.
Everyone has their own optimum temperature range. You just gonna find out by trial and error. Try to keep yourself within that range. According to Joel Spolsky, the ideal temperature is 68 and 72 degrees.
Taking a cold shower:
I noticed that taking cold showers usually picks me up when I feel like I am reaching my productivity trough, especially so when Ive gone through days without taking one.