There's a couple of things you can do. A lot depends on the set up of the systems though.
This is all based off of playing on the hardest level of the game with the largest galaxy. (I like to have to hunt for those rebs a lil bit (g)
First off, if you have a heavy reb presence in the system around your home planet, get ready for them to be jumping in a lot to attack with usually two fighter carriers.
However, they won't invade your home planet (make sure it starts with two sheilds). I just moved my ships off to attack their planets when they were jumping in to attack Courusant. Which would also disrupt their troops being sent to renforce those bases.
Make sure you have Vader and the Emperor recruiting heavily since you only have those two, while the rebs have Luke, Mon, Liea, Han.
You need two things really fast while your out recruiting and hitting systems with diplomats that don't have a heavy reb presance.
First, even though it takes a bit, have a planet that's pure construction yards. This will hurt a bit (or seem to) as the rebs start cracking out troops and ships. Don't worry about it. Just keep your fleets jumping around if you have to and you don't have the ship yards.
As soon as you have a planet with full contrustion yards (with two sheilds) then start building a super ship yard.
What I usually look for in planets is one super contruction yard close to home, but not on it, and one ship yard. Contruction yards I'll build up on planets with around 10 in energy (or better). Ship yards I always reserve for the 14 in energy.
During this I am usually building up my characters esp and sab. That leadership on those imp characters rock. I run em in sets of two on main characters (one working, one shield). They rarely get caught once they are over 90. So getting them built up in the begining is important. I'll send two groups in at the same time to sab any troops, then run one back on a esp and the other group back on an uprising mission.
This gives you a couple of choices:
1) If you happen to have a ship in that sector, you can stop troop renforcements by poping in and out. Using esp to know when they are comming and sabing any ships they try to send in to project the planet.
2) If they are sending in someone to general keep sabing the troops with your other team and then kidnap the general.
3) They send in a diplomat and if your lucky it's Leia or Luke (evil grin)
You will however have the rebs attention in that sector which will take some slight pressure off your others for a time. During which of course your cranking out your contrustion yards, finishing ship yards and have your people recruited.
Once the Emperor is done recruiting I always send him esp on the home planet over and over. That keeps the leadership up and well, I just like to hear him whine when he fails (g)
Since the Imps are so resource heavy and their ships take a long time to build, I usually have three super ship yards. It depends on how the game is going, however, when I start trying that.
If the rebs are whomping me space wise, I crank out esp driods, set them on main planets, get sheilds up then start building super ship yards.
If the rebs aren't whomping me space wise, but instead taking chucks out of the map with diplomats, I make a ship yard planet with about four ship yards on it, and start entrenching in on my home planets (sheilds) while building up troops to hold them down plus any new planets. Then I go on a mass sab rampage in the system they are hitting hard. With the sab, esp, uprising thing.
If I don't have a planet in that sector or a diplomat I can spare to take one, I just put them on a ship and stick them over a neutral planet that's close to the one I am going to hit.
Once I have the super ship yards, I start turning out tie's on one, light cruisers on another and my captial ships on another.
Pretty much once you have that, you've won. It's just a matter of time. (rubs hands evilly)
To keep your head above water in the begining, don't worry to much about having alot of troops on your planets. They take to long to build for the Imps at that stage and you'll struggle longer if you try to build troop barracks vs. contruction yards. Instead build en mass esp driods and sab crews. Esp your own planets and sab the rebs as they run in to hurt you.
Also, any planets with super contruction yards are going to be a BIG target for the rebs. They try to hit those suckers shields time and time again. I usually run two esp driods on those planets.
Hope that helps some