All you need is configure Virtual Servers under NAT Setting (something like on the fig. 1). In this case I have FTP, SSH accesses to my FreeNAS machine, which have internal IP address 192.168.1.250. Beside that I can control Bittorrent service by Web interface from anywhere on the Internet.
Fig. 1. Virtual Server setting on WL500g.
If you don't have static IP address, so you should try http://www.dyndns.com/ (free account is enough for the beginning).
The next step should be security realization...