I would offer a bit of what I know should seem to be obvious advice:
Please pay careful attention when using megaprims that you do not 'bleed over' into your neighbor's region. I have left a message for someone that has several gigantic flattened, scripted spheres they are using for a submerged water effect. 2 of them come nearly halfway over into my region and it makes for very interesting region performance fluctuations.
I've also noted that there are several of these prims spilling over into other people's regions on the far corners although they may be empty at this time. No exuse for shoddy building practices. Accidents happen, especially when you have a scripted obect that gets away from you, but we should all endeavor to respect each other. I certainly don't mind sharing some space to allow someone to 'finish off' a build by building some on my region so it looks right, but as most of us know - overlapping sim borders especially with scripted objects often times doesn't work so well