Depress the follower with a pencil (or like). Put a small punch or coat hanger thru the witness holes on the side of the mag tube just UNDER the follower. This will hold the spring compressed. Shake, rattle and roll the mag until the follower drops out. Remove your coat hanger carefully (watch your peepers!), and the spring should pop up and out.
Shane's got it, but the only thing I'd add is to use a pen or other blunt doodad to depress the spring for loading and unloading it into the mag tube. This is an easy way to prevent the spring from becoming part of the ceiling or your eye.
I'll go one further. On some mags the follower will not shake rattle and roll out so I keep a piece of coat hanger w/a sharp bend at the end to hook the follower and pull it out. Of course it should go w/o saying but don't force it.
Try using a wood dowel (preferably square in cross-section, nearly the same width as the inside of the mag body, by about 1/2" in fore/aft length), to both compress the spring and follower initially, and to re-compress the spring for re-assembly. Insert this dowel into the rear portion of the mag body (aft of the feed lips) when re-installing the spring, and it will eliminate the capability of the spring from launching. Also, it makes it real easy to insert a rod through a witness hole just in front of the dowel.