Mostly I go into the fabric store, find one print I like and then find others that go well with it. That method involves a lot of picking up, putting down, standing back and squinting. If the blocks at the end look a little off, a contrasting border has always set things right.
Something nobody else mentioned is that before you go to the fabric store, print out a whole bunch of of your block pattern, find that big box of crayons and work out different color combinations on paper in order to get an idea of what to shoot for.