great question! I only have gnomes in my garden so far, and I am forever re-erecting them
I too would love to find out how to keep them from falling over. I plan to add fairies this year...or next I imagine they would have a much small base than the gnomes I currently have.
For the mini garden structures we have made, DH just embedded spikes in the bottoms of them, I guess that would be similar to your nail idea.