Do you have any tips on how to keep a fresh sense of perspective towards the production when you've heard the songs day in and out over many, many months?
One thing I try to remember is to trust my instincts when I've been sucked into a long and deep black hole. It's very easy to second guess yourself the longer a project goes on. I try to commit to things (arrangement, efx, tones etc) if I have an initial response of "this is cool!" Usually if I don't commit, then maybe I wasn't quite sure of the idea in the first place.
I also reference demos, especially if there was something I really liked in the original version.
When in doubt, if I'm struggling with something, it's time to "drop tools" and either go home or work on another song. Then, the first thing I do the next morning, over a cup of coffee, is listen to the ruff mix from the night before and write down my comments. It's funny after a night's sleep how clear your brain can hear a song!