Basics: The United States in Axis and Allies has to fight a two front war.  First and foremost, you must ensure that Britain and Russia can handle Germany.  This might mean you have to invade France or Italy, but if they're doing well you can focus on Japan.  You must always be aware of the Yellow Dragon to your west, otherwise it will kill you.

Here come the Doughboys!
By Master Sergeant Beutin

    In order for America to make the biggest contribution, first make an effort to draw the Japanese in close to the West COast and then slap his fleet with your air force.  In the East, build a transport every turn or more and as much aerial equipment as possible.  After Britain and Russia are on their way to defeating Germany, crank out the ships in the Pacific and beat-up on the Japanese from here to Tokyo!