While this article focuses on short stories I found this usefull for my long stories as well.

How To Avoid Too Much Back Story

By Ginny Wiehardt, About.com

Do your short stories tend to get bogged down in back story? Do certain scenes seem to drag, even to you? As Alicia Erian pointed out in an interview for this site, screenwriters have a lot to teach fiction writers about creating fiction that moves. This writing exercise will help you take advantage of these lessons to create forward-moving fiction, thinking of a scene visually, strictly adhering to the present moment, to eliminate unnecessary back story.

(You can find other exercises on craft here.)

Here's How:

1. Choose a scene from one of your short stories or novels that seems to drag. Scenes designed to be more action-oriented are particularly well-suited to this exercise.
2. Rewrite the scene as a play or screenplay. In other words, tell the story using only dialogue and brief descriptions of action and characters. (If you aren't familiar with screenwriting or playwrighting formats, don't worry. This isn't an exercise in formatting, but in thinking visually.)
3. Practice economy. Think strategically about how character can be revealed through action and dialogue. (Syd Field has excellent examples of how this can be done in his classic book, "Screenplay.") Instead of telling the reader what a character is like, find a way to illustrate character as the plot unfolds.
4. Rewrite the scene in prose, abstaining from back story and long descriptions, and incorporating some of the details you have added in writing it as a screenplay.
5. Take a few days off from the work and return to it later, noting how the pace of the work has changed.

Tips:

1. In some instances, backstory will be necessary to the plot of a story. Determine what's absolutely necessary and what the reader can surmise from the dialogue and the action. Readers generally pick up on and remember more details than you might expect.
2. Don't confuse foward-moving fiction with fiction written for the screen. It's possible to write rich, literary work that also has movement.
3. It's easy enough to reinsert any necessary information later. When you start to get feedback on the work, people will let you know if anything is confusing.

What You Need:

• A story or novel written within the past few years.
• A pen and paper or a computer.