On the plus side, putting down rebellions is a quick way for your ground troops to gain experience before you attempt to conquer another empire. I often leave only a few troops on a recently conquered colony in the hopes they will rebel. I leave enough to put the rebellion down, and if I'm lucky, the colony will rebel two or three times. Each time it does, my troops there gain additional experience. One of the main Wonders I try and build is the Troop Cloning facility. With that facility you can create some very powerful invasion forces that are relatively small in number (less maintenance costs). If you're not one of the more militant species, it's really great if one of the first independents you conquer is one (Mortalen, Naxxillian, or one of the bugs). You can incorporate them into your invasion force. Not only do they start out as more powerful per unit, but their maintenance cost is usually much lower. For even easier experience gain, I really like it when there's a pirate base on the newly conquered colony, too. I only leave a large occupying force on conquered colony if I'm at war with that colony's Empire. Then I'm not after experience, I'm after gaining and holding territory.
I once heard there was a garage, which some said was air tight.