As above, make sure you have a good understanding of the rules, plus the house rules you have, if any.
I make flowcharts and cheat sheets for myself on the house rule tables and charts, and some of the official ones, too, and have them taped up to my own four-panel judge's shield. I also put together a player's guide manual that has copies of al the maps, house rules, important tables, world and subsector lists, bios of important NPC's and/or major officials and any other thing I think they might need on the fly. I'll include deckplans and full descriptions of their ship if they have one. And it's a binder so I can add stuff as we go along.
I have a Range Band chart for grease pencil and minis that has ranges, rolls, and other info on it ready for space combat. Personal combat is more free-flowing but I have a range banding chart in my cheat manual.
Write a broad story arc for the campaign with enough info you can keep the Big Picture under control and ticking along while the players do what they want to/have to. For consistency's sake at the very least, and something to nudge them back into involvement in if the circumstances warrant.
Write up a few 1-2 session type adventures to have ready for a lull in the action, and maybe a few for looking ahead to when they reach planets or whatever you know they are headed for, or to plug in if things go someplace you didn't expect.
Have some quick cargoes and passenger list ready if they start with a ship.
Don't be afraid to ask the players what they want to do to give you an idea of how to color some of the campaign. Have some NPC's appropriate to those ends ready to insert here and there.
Most of all just be OK with winging it because players will always surprise you, or even just because sessions might run for an unplanned amount of time.
Finally, it's a shared imaginary space and the ref gets to play, too. If you aren't having fun because you're overloaded then pare back the scope of things. It might be time for something like a barfight or "Help the colonists fight the baddies" simple adventure to help bring the focus into macro.