Working on more than one paintings stops me getting bogged down on one particular thing. Making a concious effort to work in small blocks of time on a painting (1/2 hour is usually a long enough time to spend on one painting) allows you to approach each new pass with a fresh eye.
Working on several paintings also allows one painting to inform the other and grow ideas. Here are nine paintings I am actively working on at the moment - but the pile of unfinished paintings is much larger. This no longer bothers me like it used to, another benefit of working on more than one at a time.