Assuming 30-rd mags, I have always used a small, flat screwdriver inserted between the mag body and the floorplate.  Twist screwdriver just enough to allow the punched depressions in the floorplate to clear the body of the mag.  Slide floorplate off carefully, so as not to lose control of the spring.  Straighten floorplate by hand if req'd.
20-rd mags.  Use the bullet tip of a 5.56 ball round.  The diameter of the hole in the floorplate is [i]supposed[/i] to limit the insertion of the bullet tip to the correct distance.  Common sense prevails, of course, and watch that spring!