[quote="namelessfly"]The old Two Province system would work.
Recruit troops from one planet to garrison another planet.
Fear of what will happen to their home planet if they rebel or fail to maintain order will motivate the troops.
The French were doing this in the 18th century and earlier. They would raise regiments from border provinces such as Alsace-Lorraine or from ethnic groups within areas they had aquired such as Tuscany, Dutch provinces, Spanish etc. The units would be posted to other parts of the country where they were they could be (and were) expcected to function as local garrison and deal with problems such as local uprisings and general police work. While they would share French as a military language they usually retained the local language of dialect of the original home region and were "different" from the local population they were stationed in. France also made good use of what were effectively foreign units raised from German Principalities, Swiss Cantons, parts of the Austrian Empire etc. The same idea held with these units, put them where you might need troops within France that would not side with the local population when it came to enforce the laws or royal demand. You find units sent over to America durring American Revolution such as Dillons which was raised from Irish expatriates (some children and grandchildren of earlier Irish fighting in French units against the English) and Luzon's Legion (mixed force unit of Horse and Infantry) raised primarily from troops out of many European countries.
You should also consider that in those centuries, what is considered mercenary work now was a common practice both for officers and men. The reasons varied but included looking for advancement (typically the officers from smaller countries or who had "problems" at home) just needing employment or had to/needed to leave their homeland for things like politics, religion etc.
