I started shaving my nuts by trimming down the bulk of the hair with a normal beard trimmer and then taking off the remaining stubble with shaving cream and a 3 blade shaving cartridge. Over the years, I've tried using my rotary shaver, but find that it tends to pull and nick occasionally.
A few months ago, I found the Phillips Norelco Body Groomer:
http://www.shaveeverywhere.com/
It does a great job on my sac and shaft. Also comes with three guards that allow you to trim hair to different lengths. Besides my sac and shaft, I use it on my chest, armpits, and anywhere else that needs to be trimmed...
Available at Target and other similar sources...