Thanks, I'm glad you like it.
Firstly, I don't feel like it's that good; people seam to like it for some reason; I'm really not sure why. I mean I see I'm improving but I'm nowhere near people who draw professionally (most likely never will be). Makes sense I guess, they spend orders of magnitude more time on drawing, probably since childhood or something.
As for your question: I recommend* 3 things (assuming you're a beginner) that work for me:
1. steal** palette from some pixelart you like (the less colours the better, 4 is nice number but anything from 2 to 8 should be good to start with)
2. use a small sprite size: 8x8 or 16x16.
3. if you're unhappy/tired of something just save it on your hard drive (or wherever), forget about it, discover few months later and release with some tweaks as "not that bad after all" - this might be a terrible advice, but this is what I do (might be the reason why I never finish anything) :)
* - also I guess asking an actual artist might be better idea - I'm just a programmer who sometimes draws;
** - stealing is good (don't quote me on that :D), also: https://lospec.com/palette-list