The first rule is: design games like adventures. It might sound simplistic but novice and long-time Game Masters alike have stared across the table wondering why their players seem bored. This is actually quite common as the assumed far trader model, combined with the enormity of a sandbox the size of a universe, can easily become overwhelming.
The second rule is: stories are the life’s blood of a Traveller game. Before you get started on how you much prefer the sandbox method of playing, let me stop you – stories are essential for a Sci Fi game no matter how you craft them. Watch an episode of Star Trek and tell me you don’t see larger stories at work regardless of what choices the crew of the Enterprise makes; this example also serves well as it demonstrates that any good game amongst the stars should have multiple over-reaching arcs.
The third rule is: don’t build dungeons. This may sound silly, but if you started gaming as a fantasy player, particularly of the D&D variety, you’d be surprised at how easy it is to fall back into dungeon designing. Before long, every adventure is populated with searching and destroying nests of errant belters/mad scientists/pirates/renegade pop stars in their base on the asteroid/orbital station/corsair/tour bus; sure, they may be scientists instead of orcs, but you’re right back to moving through a relatively static environment and clearing rooms.
The fourth rule is: populate your world with personalities. There are no monsters in Traveller. Instead, the bad guys are just that – bad guys. They have motives, reasons for being, and a history of how they wound up in the player’s way. If you treat them as such you not only wind up with a more believable plot but your players will likely have to use multiple approaches to solving problems that go beyond drawing their snub pistol.
The fifth rule is: life is messy. There’s a reason Classic Traveller and Mongoose Traveller have malfunction charts and encounter tables. Use them. You don’t need to overdo it but semi-regular chance happenings are part of life. We never know who we might bump into in the grocery store or when our car might breakdown; the same should be true with your players. I enjoy keeping track of these things and letting some of them become persistent problems throughout the game. The pushy guy at the Down Port on Tavvol III is always the one to greet the ship; and why does that hydro-seal keep breaking on the comm manifold every couple of weeks?