Chimneys should be cleaned and inspected at least once a year--more frequently if creosote is building up. When you have your chimney cleaned, have your dryer vent cleaned out as well. Lint build-up in dryer vents is a common cause of residential fires.
Excellent information about chimneys and their maintenance is available on the site of the Chimney Safety Institute of America
They also have a lot of videos
The following local chimney sweeps come recommended:
John Gray (a Captain in our neighboring Orland Volunteer Fire Department and a career firefighter with the City of Bangor) operates as Riverside Chimney Sweep — he will inspect and sweep chimneys, install stainless steel liners and clean dryer vents—all fire-prevention services. John is certified and fully insured. You can contact him at 949-0781 or email@example.com.
Able Chimney Company, LLC., (207) 947-2244 is a well-established company out of Bangor. Complete information about their services is on their site at www.ablechimneycompany.com