Especially in SF games, PCs generally speaking have starships and one of the obvious attributes of starships is that they go places. In some settings a campaign will have a dozen to 30-40 worlds. Characters could potentially go to any one of them. In D&D olden days people were warned against giving character the access to flight spells and items because it made places too easy to access. You were supposed to travel on the roads and get jumped darn it!
Sooner or later you have to wing it. Players are not very responsive to the carrot and stick approach and will go off course to some dumb rock you rolled the stats up for and promptly ignored. What to do?
1) Stall! Starports, space stations and Jovian gas and goes are pretty much the same all over. Think of some way to keep the group at one of them until next session till you can
2) Randomize. There are a lot of charts and tables out there for whipping up a world on the spot. Just don't take this approach too far. One or two hooks to jog your imagination will be a start.
3) Make Your First Shot Fast And Your Second Shot Good. Find some clue or detail to remind them this place is a new to them. Jumping from a world of bolt action rifle wielding nomads to a garden spot where robots walk cybernetically enhanced pets ought to wow them enough for you to make some more stuff up while they gawp. The internet has a lot of SF artwork you could use to set a scene for your group. Use it.
4) Six Points. This was a little game I played a while back. Look at the six characteristics in the Classic Traveller format and its successors and figure out one hook for each characteristic. If they're tied in in some way even better!
5) Blatantly Borrow. I wouldn't call it stealing unless you're making money off it and trying to hide the fact. Cliches can work because they are familiar territory and familiar territory is always a good place to start in introducing a new world. Model your new world on another or an amalgam. A desert world with fierce nomad raiders shows up in a lot of universes. Of course after doing the initial borrowing you owe it to your players to come up with a twist (next session though).
6) Practice. This is not so much directions as a way of life. The more worlds you create the easier it becomes and the quicker your can do it. Write up a few worlds as quick as you can.
7) Gradually Amp Up the Weirdness. You don't have to work up a dozen strange and wonderful features the moment your characters have boots on the ground. You can introduce local oddities gradually as your group moves into the next adventure.
8) Use the Players' Ideas. The World is Myth etc. Use the buzz talk against them. When a player comes up with a speculation or theory about their surroundings that sounds good (or at least better than what you've got) use it! They'll be thrilled to be right. Just don't make it too obvious or the speculation may turn to why the streets are paved with gold and people with amorous, gorgeous natives.
I'd be interested in hearing more tricks from the audience.