Doodads are something that really requires practice. You can try holding control and using page up/down and messing around with that. I believe there's also an option to place a doodad ontop of another if you hold shift and click while placing.
While I do suck at terraining myself, I should mention that shift+clicking just removes most of the collision of the doodads, allowing you to place them very close to each other, or in each other.
Note however, that you can only place them. You cannot move them after placement if there is any collision interference from another doodad.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.