Forum Info Dump vs. Insufficient Information

How do you avoid anything that could possibly come across as an information dump without withholding important details instead?

This is a problem for me.  I tend to err on the side of caution, leaving things out rather than explaining details that aren't important to the story I'm telling at the time (Does anyone really need, or want, to know that the captain of the Excalibur lived in Tennessee at one time?), but then I have readers telling me that they got lost because they didn't know what was going on... And when I do give more detail of setting, character backstory, whatever, it falls flat.  I don't think it's something as simple as my narrative style; I'm generally good at that.  The fact that I can't just assume that readers already know the setting - I write sci-fi, after all - doesn't make matters easier.

Some readers have told me that I drop too many names in my stories; others have told me, concerning the same stories, that the mention of people and places outside the events of that particular story is part of what makes the setting believable - that it gives the feeling that there is more to the world than has been told about yet.  So I don't know.