Check here for all the details, what voltage to use (check out the calculator at the bottom), using a filament transformer, as well as the variac info about halfway down the first paragraph: http://www.unclespot.com/speakerbreakin.html
Thanks guys, thats some great information. If the calculator is correct though, it would only be practical to use the variac for lower wattage speakers. Using it on a speaker over 25 watts would require a pretty heavy duty unit.
It's actually not that bad. For a 100 watt, 16 Ohm speaker, this works out to be something around 23 volts for the 1/3 power break-in (if my memory serves correct). My Staco variac, which I use for my amp and speaker break-in, gives continuously variable voltage from 0-140 volts. So this is more than enough. I have broken in many a pair of g12h30's and V30's using the variac procedure. Just make sure to check the voltage using a digital meter to make sure you have it accurately dialed in.