This is a messy job so make sure you put down plastic drop cloths and wear old clothes and rubber gloves to protect both the area surrounding the fireplace and yourself!
Use a spray bottle to wet the bricks throroughly. This will help to loosen the soot.
Pour some powdered dishwashing detergent onto a plate and have a bucket of hot water close by.
Dip a stiff bristled brush into the hot water and then into the detergent so it sticks to the brush.
Scrub the bricks. The paste formed by the detergent and the water is a strong concentrated cleaner that will cut through the soot.
Depending on the amount of build-up and how long it's been there you may have to repeat the process a few times. The TSP ingredients in the detergent are what will cut the soot.
Once the area is completely cleaned, rinse it will with hot water to make sure that all of the detergent is gone.
Just look at the drop cloth to see how much soot i removed, this fireplace was really bad and look at it now! I'm Mrs. Fixit and its just that simple!