Jonathan Blow has a great talk on long projects (The Witness was like 8 years in development?) here which I watch once in a while: https://www.youtube.com/watch?v=d0m0jIzJfiQ

Some highlights:

- He takes a week long vacation every now and then to hack on a completely different game idea for fun, with the focus on it being short and fun

- Time for relaxation and unstructured thinking

- Walks / showers can produce amazing ideas