Here's my favorite strategy for not having to defend my empire. I like to take as many planets as possible and build only a few stuctures at each planet (i think you are supposed to do this as advent). then if the pirates come, i just scuttle everything, including constructors. I can usually take most of the map before the computer does with this strategy. Unfortuntately, they usually end up making as much money as I make anyway, just because the computer is lame.