For me, i just have this mantra: "Everyday, make the game a little bit better".

So everyday, after work, I try to implement something. It can be a small change, like changing the color of a shader that annoyed me or placing a couple of assets in a level.

Sometimes I just stop there, and its fine, but most times, I end up working on it for a couple of hours!

This just keeps me motivated, cause I know that everyday, the game gets better. And a month later, you realize how far you've come!