I was playing a small map today just against the computer, and thought of something that seems to work well for me.. I do the usual thing with building repair posts and stationary defenses to ward off pirates and siege groups, but I also found that if I post a few frigates at that planet it makes it about 300% more effective. Tricky part is figuring repair coverage for them and getting them to stay *in* it. Also I have to remember to turn auto-join fleet off, otherwise passing fleets have a tendency to scoop the pickets up.
This certainly won't stop a determined attack, but it seems to work well for those pesky siege groups and pirates, and is great when I need to call in reinforcements ASAP. Of course, in the long run, fleet upkeep would be a pain.. but I can cross that bridge when I come to it.
Thoughts?