Deep, the trick is to use natural barbed wire. Holly trees or pyracanthus plants, or even brambles if you can get them to trail along the top. The best bit is they look "decorative". Barbed wire to me suggests someone has something worth protecting / nicking (and is easy to get over just by throwing an old coat over it) - holly just looks like they want a hedge. Spiky plants are recommended by the police, I asked them about it for my house 8 yrs ago.
Over here we use bourgainvillia - wooden thorns an inch long
Brendan