I find a lot that being around people helps. Go to a hack space (e.g. https://www.meetup.com/HackWimbledon/) or spend and evening with a friend who's working on their own side project. My biggest problem when working on side projects is that Netflix is only a few keypresses away, so if you put yourself in a situation where those distractions are impossible, it really helps focus and makes the best use of your limited time.