All I have to say is "support, support, support"!! Make sure you put in a cake board between every two layers of cake and have a sturdy internal support structure!
My cake decorating tutor freezes the cakes prior to carving, but I haven't tried it myself yet. I've got a sculptured Iggle Piggle from "night garden" to do next month so I'll be trying it out then. Keep us all posted on how well it went, cant wait to see the result
Freeze for at least 30 minutes. This really firms the cake and makes it nearly without crumbs when you carve. Good Luck!!!!