This is to cool! Here we are talking about DC drives, I look to the right, and see a Google advertisement for my company! Sorry, I just had to mention this.
Goose is on the right track, use a SCR DC motor drive. KB is one of many brands.
The SCR DC motor drive has a built in current limit circuit, speed regulation, IR compensation, uses a potentiometer for speed command, simple switch for run/stop command, etc, etc.
The Variac, and light dimmers aren't used in industrial applications because they do not protect the motor, do not provide good speed regulation, and require breaking line current vs. just shutting off the SCR's gate pulse. The Variac will work, but I wouldn't use it.
In short, the SCR DC motor drive has been around since the SCR was first designed (50's?), and is the proper tool for the job.
If I did not work for the company I did (I have a few DC drives of my own), I would search EBAY for "DC SPEED CONTROL". Most of the hits are good, but I would stay away from router controls, and the PWM stuff. The simple, rugged, industrial DC drive is what you want. Post your link here, and I'll be happy to check it out for you.
Hope this helps.