LaggyFox shared good insights.
What comes to code. I always try to write code to be super modular. For example I always use the same Audio and Animation manager in my projects as they just fit to any project. Also the data structure in the projects tend to be always the same, so I don't have to design it from a scratch.
For art, in lot of cases I use ready made color palette from https://lospec.com/palette-list