Geoff Dyer, in his 10 rules of writing says:
Have more than one idea on the go at any one time. If it's a choice between writing a book and doing nothing I will always choose the latter. It's only if I have an idea for two books that I choose one rather than the other. I always have to feel that I'm bunking off from something.
Do you think it's better to have a few things on the go or to focus on one project at a time?