In addition to having annual professional chimney sweeps, you should give your fireplace a thorough cleaning at the end of the season to keep it spotless until the next year.
Supplies
Cover surrounding surfaces with newspaper or a drop cloth, and wear a protective mask to avoid breathing in the particles.
Trisodium phosphate (TSP): Long the cleaner of choice for tackling soot, TSP comes with many drawbacks—it can be harmful to both you and the environment (by entering waterways). When using TSP, take these extra precautions: Work only in a well-ventilated area; wear gloves, long sleeves, and pants to protect skin; and dispose of used sponges or cloths.
For a greener cleaner:
Mix together a paste of cream of tartar and water and apply as you would TSP, or combine equal parts white vinegar and water in a spray bottle. You’ll have to do a lot more—and harder—scrubbing to remove all the soot, but you might find that the better option.
Hire a professional chimney sweep once a year (ideally at the end of your fire-burning season) to manage creosote, a thick brownish-black residue that can build up on the inner surface of the chimney and is a major cause of chimney fires.