I have 3 things that keep me from sideproject burn out:
1. Users. Once someone uses your project and you're growing, even if it's slow, it "motivates" me to keep working on it. That is also why for me, it's a must to keep the project small and launch ASAP, which in turn also helps against feature creep and bikeshedding.
2. Delegation. Once I validate the project (people are using it) I try my best to outsource time consuming tasks, mostly programming, administrative, research, and data entry. This frees me up to enjoy leisure time while not feeling the project is stagnating.
You have to find your balance - decide what's the velocity of the project you're comfortable with. Once I realized that wanting to build and grow as fast as possible is stressing me out and ruining my social life, I decided I'm going to try and be ok with slow growth since I don't depend on it for a living.
3. Part-time job. I work 3 days a week. This should've probably been number 1 on the list. I still make good enough money to live comfortably, while my side projects make little money I can save. Obviously not everyone can or want that, but as a developer I have the privilege of earning the same as some of my less techy friends with 60% of the work. Frees me up for a whole bunch stuff.
This might not apply if you have kids and want to save as much money for their futures sake.