Let me start by a shameless plug.
I've written an article on Gamedev about how to remain motivated during indie development.
I read that when it came out! Tried it actually, kinda worked, until my normal motivation killers came >.<
Keep away what can ruin your motivation: client (if it's work-for-hire).
No matter if it's for a leaving or an hobby, detach yourself from your project sometimes! It help to come back at it with a fresh view.
Show you project! Feedback can be really motivating and give inspiration
It already has been said but working with good group is more motivating than anything!
I like the list you got there!
There are a lot of highly unmotivated experts out there.
Okay, I guess I was Assuming a bit TOO much wasn't I? Whoops!
Anyhow, Thanks for all the replies so far; Keep them coming! I hope everyone who reads this learns a thing or two (including me >.<)