Print out your poem. Fax yourself the printout. The fact that you wrote the poem establishes copyright all by itself, according tho the US Copyright Office, but printing it out and then faxing it to yourself gives you a definite paper trail, because the the fax date will establish that the poem was in your possession at that time.
Write the poem, print it out, fold it and stuff it in an envelope. Then mail it to yourself. Don't open it when you receive it. Also known as the poor man's copyright, this technique is legally valid, uncomplicated and provides you with a dated documented paper trail that will establish your authorship. The envelope you mail it in will be postmarked and, thus, establish a date of possession.
File a copyright notice with the United States Copyright Office. Use the Electronic Copyright Office (eco.copyright.gov/eService_enu/start.swe). Create an account, then fill out the registration form. Pay the fee by credit, debit card or electronic check. Indicate whether you will deposit your manuscript electronically or via mail. Upload your manuscript file if you are depositing electronically. To mail your manuscript, print a shipping slip and send a hard copy to the U.S. Copyright Office (copyright.gov; 101 Independence Ave. SE, Washington, DC 20559-6222).