Very simple little exercise, which I personally love, and find rather consoling and calming after a mental bus journey home.
Step 1: Go to this site: http://www.rainymood.com/
Oh my, isn't that some delicious white noise there? I love the sound of rain on my window as I work in the comfort of a dry studio. Don't really know why...
Step 2: For some added class, play this on youtube at the same time: http://www.youtube.com/watch?v=HMnrl0tmd3k
(Step 2.5: Another great song to play parallel; http://www.youtube.com/watch?v=nlu2z2gkhhI )
Step 3: Grab a sketchbook, coffee, glass of wine (all of the above?), sit back, and thank whatever higher power you believe in that there's no people around to ruin the mood.
Repeat as necessary.