Comment your code, use groups, organize it well. I prefer to have only 1-2 event sheets but with many groups. it makes searching much easier.
Don't do many events assuming it works .
Test after every minor implementation.
Every 30-60 minutes comment your code or tidy up.
I save incremental every major modification (every 1-2 hour) Eg: GameName_v1.capx, then GameName_v2.capx ... etc
Enable the build-in backup for both local hdd and dropbox. It will save you ward work in case a UFO blasts your PC
Learn to use the Debug option in case you don't know it
Use functions if a group of events are repeating more then 2-3 times in your game. It will only need to make 1 modification instead of many.
Make the code as general and flexibile as possible. For example, if you want to position an object in the middle of the layout, and the layout is 600 by 300, don't position it at 300 by 150, but instead position it at Layout.Width/2 by Layout.Height/2 (that is not really vital, only if you want to modify the positions and sizez you will have to modify the values)
These are just a few that I can remember right now that become an habit for me