Your chimneys should be inspected at least annually.
If your inspection determines that the chimney needs to be cleaned, you may use a chimney brush, and material to seal off your fireplace or base of chimney from allowing soot into your home. There are also chimney sweeps who can make short work of the job.
According to the paper titled: Home Fires Involving Heating Equipment Fireplaces, Chimney And Chimney Connectors, by John R. Hall, Jr., January 2010, Half (51%) of home fireplace, chimney and chimney connector fires involve failure to clean as a factor contributing to ignition.
The complete report may be viewed at http://www.nfpa.org/assets/files/PDF/OS.fireplaces.pdf