How do you avoid network conflict with VPN internal networks?

We have several IPSec VPNs with our partners and customers and occasionally run into IP conflicts with their network. The solution in our case is to do either source-NAT or destination-NAT over the VPN. We are using Juniper Netscreen and SSG products, but I assume this can be handled by most higher-end IPSec VPN devices.

I think that what ever you use, you're going to risk a conflict. I would say that very few networks use ranges under 172.16, but I have no evidence to back that up; just the gut feeling that no one can remember it. You could use public IP addresses, but that's a bit of a waste and you may not have enough to spare.

An alternative could be to use IPv6 for your VPN. This would require setting up IPv6 to every host you'd want access to, but you'd definitely be using a unique range, especially if you get yourself a /48 allocated to your organisation.