You can still do it even with only one public IP address. First, decide which VPN technology you're going to use (IPSec, PPTP, L2TP) and find out which port(s) it communicates over. Then set up port forwarding in your router/firewall to forward all incoming traffic on those ports to a specified machine (private IP address) on your LAN behind the firewall. When you connect to your public IP address with VPN traffic, it will end up at the VPN machine on the private network.
Be cautious if you want to use IPSec - it doesn't work too well over NAT (at either end). PPTP based VPNs should work through NAT without problems
