A great way to keep the small-scale Rebel raids down is to build two Mauler squadrons on each planet. As soon as tactical combat starts for a raid, give all the Maulers an emergency move order (double-click) just past the Rebel landing zone. If the Rebel raid consists of infantry, they'll be dead as soon as they hit the dirt.
I typically try to build a barracks and light factory on each planet, so I get some free stormies to capture build pads and some free AT-ST's to squish infantry. If money is tight, I just build the factory and I move a few squads of troopers in, but use them only for capturing build pads and keep them out of direct combat.
Here's a space trick: for each pair of planets connected by a trade route, you only need one defensive fleet. You can zip it between the connected planets before any attack fleet comes in--just keep an eye out for the "Rebel fleet incoming at ___" message. (Against a human opponent, this would be worthless, because they jsut have to split their attack into a two-pronged force to counter it.)
I like using Boba Fett on land to fly over the enemy base so I can target it for a bombing run.