Includes are good to keep things organised by catégories (ennemi AIs in one event sheet, decorative elements in another, inputs in another one, etc..)
Groups can really help subdivising things (for exemple: one group per ennemi AI), and that can be deactivated in some cases to reduce the impact on the CPU
functions are a pretty neat thing, it help not having to repeat constantly the same things over and over, or subdivising complex math calculations, which also helps if you need to modify something (having to modify it only once rather than a lot of times). local variables and constants are also pretty good to have this "change once apply everywhere in the code" effect.
My primary objective is to try to always code in a similar way, so if there is a problem, I have an idea of where the problem can be located.
The debbuger is a good tool to have an idea of what is happening, also, sometimes I read the entire event sheet, from top to bottom, to see if there is something that doesn't feels right.