Portsnap is definitely the way to go. Here is a great PDF to read over if your interested, it may take you a bit to pull over 56k.
http://www.daemonology.net/papers/bsdcan07.pdf
Also, the general guideline from my understanding is:
First Run of Portsnap
portsnap fetch && portsnap extract
Updating the Ports Tree thereafter
portsnap fetch && portsnap update