Foliage normally peaks in the northern side of Maine by the last week in September, while the fall foliage on the southern coast peaks in mid October. My favorite spots to see the fall foliage in Maine are Rangeley,Greenville, along Moosehead Lake, Baxter State Park, Mt. Desert Island, the White Mountains, and of course, the beautiful Acadia National Park.
Foliage peaks sooner at the higher elevations of western and northern Maine so you'll see the most spectacular fall foliage where it is peaking. I'd look at somewhere toward the western Maine mountains for viewing the foliage. From October 7 to October 12 is a good time. you will see gorgeous colors in Acadia and Camden specially.
Driving is truly the best way to take in the foliage. A few routes I suggest for driving in Maine are Route 1 from Bath to Brunswick, Route 3 all around Acadia National Park on Mount Desert Island, Route 17 from Rockland to Augusta and Route 27 from Farmington to Kingfield. By the way, if you are fan of Stephen King, his home is located in Bangor Maine.