1. Organize Your Project Structure
2. Use One Scene
3. Use Right Materials
4. Never Use Instantiate
For example if you are creating enemies, if the player kills an enemy , disable it and when you need to create a new enemy, reposition it and activate it back again.
5. Always Use Prefabs
6. Use Depth for 2D Games Too
7. Create Art First
If you want to start off with art you can create temporary art but make sure to create your art with the same size.
8. Keep UI Simple
9. Use Light Effectively
So keep this in mind when you create your next game.