Go here:
http://www.senate.gov/ Find your senator, using the drop down state menu. This will take you to the contact information page for your state senators.
Contact them. I don't care how. Email, fax, phone, drive up and wave, whatever.
Ask them to vote no on the Federal Marriage Amendment, currently being debated on the floor of the Senate.
Tell 'em "Bette Midler" sent you.
:)