What Sotak246 said.
Also note: The supression value is the same as the defensive combat power of the troops used. Therefore, raising the initial ground combat tech, which is rather cheap in terms of research points, can greately increasing the garrison value of your troops. In addition, build divisional and brigade HQs and put commanders with good ground combat ratings in (same goes for the batallions, of course).
All boni stack, i.e. a Garrison Battalion with a +10% commander, in a brigade with a +15% commander, in a division with a +20% commander gets a 45% boost to its combat power and therfore also to its garrison value.