Well here are some tips
Make good use of EventSheets and Includes as discussed.
Minimize use of Global variables. Two reasons don't pollute the name space and it's easier to use the auto complete if it doesn't a a lot of junk in it. Ashley has done great optimizations, but it's just good JS coding practice to keep variables declared and fewer in each closure.
Use Group based variables extensively
Use Groups as a Class for Object Orientated design
Use Functions A LOT.
Design your game in Unit Test model. Design each aspect of your game on it's own away from from your main project. Seperate what feature it does from how it's used. This will give excellent re-use for other projects later and best of all this will reduce errors and allow you to isolate errors that come up later. Also if you Unit Test your features as seperate CAPX. You can come back later to improve the design feature. Which means your new projects can benefit from your own improved skill.
Tutorials and the Manual are your friends as is the Search function.
Don't bother asking can "C2 do X". 99% of the time the answer is yes. If C2 can't do it then it's a limitation with the browser technology itself. Find out if the browser is capable of X instead. Also ask "What would be the best way to go about X"
Don't be afraid of Plugins. Many plugins are awesome and can help your work load a lot. I suggest Lunarray LiteTween to make animation life easier and R0j0hounds Paster to coalese complex images groups into a single sprite image.
Be careful of plugins. Some are obsolete right now. Some one thought to use Rexrainbow's Function plugin recently even though it's obsolete. Some plugins aren't even updated and might not even work.
Be careful of plugins. Don't rely on plugins as an all in one solution for every problem. There are a handful of developers who just rely on plugins and the occasional help fix on the forum. They aren't very good at programming as they just mostly copy/paste.
Be friendly on the forums.
Document, document document. Document as you go. If you need help then other developers can understand your CAPX because you documented it and can help faster. Also if you come back to a piece of code months down the line. You might have some funky stuff going on and documentation will help you understand that better.