Try to avoid back stories. Yes, it's good for the readers to know what's going on, but if you throw the reader into an action scene or mystery scene, don't try to "leave them in suspense" by cutting to a back story. Instead, reveal it through dialogue or, the one I use a lot, in snippets of dreams. Maybe the main character's past comes back to haunt him.

This is how you keep people intrigued, on the edge of their seats. See, suspense is already enough if you don't give the back story. The readers will be craving to know what happens, what happened in the past that got the character to that point, but not just a huge blurb about the past. Then there is nothing to hide. It's not as mysterious.

Example:

Your main character wakes up tied to a chair.

Already, people are starting to form questions. Why are they there? What happened? Who did this? Where are they exactly?

A man walks in, claiming to know them.

Now your readers want to know who is this man? Where did he come from? What is their history?

Do you see where I'm going with this? Don't blurt out a back story unless it's ABSOLUTELY necessary; usually it's not 95% of the time.