This is what I do: http://i.imgur.com/ny0ltfr.png
The left-hand window is my JOSM editor. The right-hand window is a similar area in the real world (viewed in OSM) and I'm making sure that both are at a similar zoom level.
In this case I'm creating FunLand Khaiwoon, using Hong Kong Disneyland as a model. By "model" I mean I'm trying to make sure that the buildings are roughly the same size, the parking lot is roughly the same size, the little car race track is roughly the same size, the theme park as a whole is roughly the same size, etc.
This seems to work pretty well for me. I've used the same technique for airports, seaports, industrial areas, etc.
Anyone have other tips and tricks they'd care to share?
EDIT: There is now a tool that does this much more easily —OGF scale helper!