Make a regular D&D campaign and keep dropping anchors from the sky on them until someone takes a bite. Once they do, you can say "Hey, it's your fault".
More seriously, I think you can go two ways: In one you ask players to give it a try to see if they like it, fully assuming that Spelljammer is an odd setting (one of my personal favourites, I might add) and that it can be fun precisely because of that (I usually describe SJ as "It's the D&D they would play in Flash Gordon"). This route requires expecting players to take the setting in a comical fashion, which I think is fine considering Spelljammer does have a very light-hearted flavour to it, but one you can handle as the story unfolds.
A second way would be to sneak Spelljammer into a game, slowly adding elements until you can safely put your boxed set on top of the table. I think this is the easiest if you first make a pass through Planescape, since the two settings click together very nicely in their weirdness and sense of ultramundane adventures. For some people, going right into "D&D with spaceships" might be a bit jarring (particularly these days, when there isn't really a big equivalent in pop nerd culture to tempt them into trying the sub-genre. Back in the 80's and early 90's we had everything from Thundarr the Barbarian to He-Man to Flash Gordon), adding elements at a slow pace might make the transition easier.
For instance, they might stumble upon a portal sending them to another world, where they encounter a nation suffering under the plotting of beholders; as far as they know, they are just in another part of their own world or in another plane, which is still safe ground as far as current D&D goes. Later they find it's actually several factions of beholders plotting againts each other what's breaking the nation apart, slowly finding out these things are everywhere out there and warring for racial purity and/or control of several worlds. Eventually, they have to use a strange flying ship to deal with the Eye Tyrants directly and bam, they are swimming in phlogiston and visiting stone ships shaped like dwarven heads two sessions down the road.