Use Salt
Regular users of their fireplace swear that salt will prevent soot stains and help clean them if you have them. Do this regularly and your spring clean up will be a breeze.
AVOID Household Cleaners
If you do have soot stains that have solidified, remember that these are carbon deposits so are full of grease. Resist the temptation to apply bleach as it may discolor your stone. Play it safe and just use some hot water and a stiff brush.
Once you have cleaned and sealed the fireplace, use a stone soap for maintenance every month. This will leave a dirt repellent layer on the surface (a little like polishing your car).
Now, go light that fire and enjoy the ambiance that only stone fireplace can evoke.