The ever-vigilant Will Q. has put together a remarkable and incredibly useful page showing every Senator's current stance on the Federal Marriage Amendment. Will writes:
The success or failure of the Federal Marriage Amendment likely depends on whether or not it passes (with 2/3 majority needed) in the Senate. If it does pass, it may gain momentum and pass the House; and it will very likely pass in 38 state legislatures if it comes to that, as most state legislatures are fairly conservative. The Senate, being very evenly divided (48 Democrats, 1 Independent, 51 Republicans), is the key; whatever your views on the FMA, the Senate is where they should be directed.
Note that—at the present time—email is more effective than snail mail for contacting your Senators. Snail mail to Senators is being quarantined due to the ricin scare. So use those email links!
I would add that phone calls may also be effective, for those of you comfortable with calling elected officials' offices.
Will and Josh Chafetz have done all the hard work of gathering info and presenting it clearly and effectively. All you have to do is look up your state, see what your Senators are currently on record as saying, and follow a link to send them email in support or opposition. This is about as easy as it gets, folks. Go do it.
(Note that "for" means "for the amendment" and "against" means "against the amendment.")