I don’t care what they’ve told you–you can’t show everything. Some things you need to tell.
Exposition is necessary, especially for spec-fic writers who can’t rely on their readers to already be familiar with the story’s setting. The trouble for me comes when I’m trying to figure out what information is necessary, and how to communicate it without breaking the flow of the story and boring the reader (or myself) to death.
I’ve found it useful to think of my scenes as vehicles for information. The information gets broken up into digestible chunks, and I also have to justify the existence of every scene–it has to be pulling its weight by moving the plot forward and revealing character.
Before starting a scene, I’ll write down exactly what information I need to communicate. A lesson I’ve learned is that it’s better to work this out before I come up with the events themselves–otherwise I find myself trying to awkwardly stuff the info around events that I’m too fond of to cut, but might not be the best vehicle.
Once I’ve worked out that I need to communicate this, this, and this, I can come up with a series of events that will best showcase the necessary information through action and dialogue rather than solid exposition.
Orson Scott Card has a great article about revealing information and backstory on his website Hatrack River (along with a lot of other good stuff).
An excerpt from the article:
Make sure you’re beginning the story in the right place. If you immediately have to do flashbacks, etc., chances are you simply began too close to the end and you need to let us see, in correct linear time order, the events that you’re flashing back to.
If you begin at the right place, but there is information known to the characters that needs to be told to the readers, you can often lay it in, piece by piece, right where it’s actually needed.
You can read the rest of the article here.