I use Smoothwall, but the same principle will apply :
option 1 - open VPN ports to an internal VPN server, connect from outside to the VPN server, and do your work this way. (Quick and dirty)
option 2 - install OpenVPN on IPCop and let it handle VPN connections. VPN in to the Cop box and do your work this way. (Not so quick, will need a good box as VPN do take a lot of CPU resources).
option 3 - use Hamachi or GoToMyPC to connect to an internal PC remotely, then from there to your Cop box.
I prefer option 1 or 2.
Use a good, strong username/password/login credentials in all the cases.
One last thing - whatever VPN solution you go for, make sure it have logging enabled to enable you to check on failed login attempts (usually somebody trying to haxxor his/her/its way into your network).