"How a Revolution Saved an Empire," by British General Michael Rose (free reg. required for link):

http://www.nytimes.com/2007/07/05/opinion/05rose.html

Unfortunately for Britain, he [George III] attempted to fight a conventional war against insurgents, and sent far too few troops across the Atlantic to accomplish the mission. Although they initially took New York and Philadelphia, the British subsequently failed to adjust to a counterinsurgency strategy against the “war of the posts” that George Washington adopted after his defeat at Germantown, Pa., in October 1777.

Instead of trying to isolate the rebels and gain the support of the loyalist and uncommitted colonials, the British spent much of their time defending their bases and maintaining their supply lines, only occasionally venturing out on punitive expeditions. They never succeeded in cutting off the heartland of rebel resistance in New England by taking control of the Hudson River Valley. Nor was the British Army — the finest in the world — ever able to establish sufficient security in the countryside or counter rebel propaganda. It soon came to be regarded as foreign occupation force.

Finally, the British were never able to prevent a steady flow of arms, ammunition, instructors and fighters from entering the colonies from abroad. Thus Washington, whose Continental Army was down to a few thousand fit soldiers, managed to survive the harsh winter of 1777-78 at Valley Forge and rebuild his military strength. When the British switched their main effort to the Southern colonies, Nathanael Greene, probably the most successful insurgent leader in military history until Mao Zedong, was able to wear down Cornwallis’s army in the Carolinas to a point where Washington, now reinforced by the French, was able to beat the British in a conventional battle at Yorktown.
Bob