depends what you want really, a whole host of ways to do backgrounds.
as said gradients or screen tones are quick and easy,
or you can use a program like poser 10 , Daz or sketchup or even blender to make 3d refrence pics.
You could do some perspective drawing the get what you want if your willing to take a bit of time to get there ( check out Mark Crilly on you tube for that)
I use poser to make my refrence pics or i go out with my camera and snap some shots
hence my local woods become the boobieshroom fields
personally i love drawing backgrounds.