1. Visit this link.
2. Fill out the form. (and choose your museum)
3. Wait for the email.
4. Print the tickets and bring them to the Museum you choose on Museum day (9/24/2011)
Edit: Thanks Sept1967
9/24/11 is also (Public Lands Day) Free Entrance Day in the National Parks http://www.nps.gov/findapark/feefreeparks.htm You could go see a museum IN or near a nationl park, all free on the same day