I tend to do a mass fetch of the entire package repo every 6 months or so. You can either scrape the packages off
http://pkg.freebsd.org/ (used to be easier with FTP) or you can use
pkg fetch -a
to grab everything.
https://www.freebsd.org/cgi/man.cgi?query=pkg-fetch
Once you have a big directory with everything in it, when you want to install something, just cd into the directory and run:
Dependencies will be resolved automatically and offline. I don't think so many people use it like this (kind of like pre-pkgng), but it works well offline and I dislike being tied to the internet (worst part about UNIX and Linux in my opinion).