I'm not sure if Ted still does it, but if you ask, he will break in the speakers for you.
You can also break in a speaker quickly with a variac:
I like the looping idea mentioned above as it's super easy if you have a loop pedal, but I'm sure it drives the neighbors nucking futs.
What I did for my last pair of speakers, is bolt them face to face, then I wired them up out of phase to a variac (use the link above for info) and had them push/pull eachother. I just tossed them into the back of a closet and checked on them every few hours to make sure a cat wasn't sitting on them or they didn't burst into flames or anything weird.
I have some blue dogs that when from good to HOLY CRIPES in just one day with a good workout. I have a set of WGS Green Berets I need to do this to next.
On a simliar but slightly off topic note, I just received some WGS Veteran 30's that were well broken in by the previous owner. WOW! They sound so warm and woody after being broken in...*way* better then a fresh pair. you can hear the stiffness has just been worn away!
Also, IIRC, I think Scumback speakers are broken in on a variac. Jim will correct me if I'm wrong.
I have broken in speakers by connecting them to my stereo. I just threaded the bare wires of the speaker cables around/through the guitar speaker terminals, cranked the radio and went to work. Did this for a couple of days.