Az yashir... va'yomru leimor. Why the extra word "leimor" -- it should be enough to say they sang and va'yomru, here are the words to the song?
The Tiferes Shlomo in many places says a derush on "motza sefasecha tishmor v'asisa ka'aser nadarata." A person might be moved by some event or some cause and makes a pledge of a korban to the beis ma'kikdash, or in our times makes a gift to the shul or to some charity. More often than not, between the time of the pledge and the time the person reaches into his/her pocket to write the check, some of that good feeling, that hislahavus, that desire to give, has worn off. The Torah warns us to be careful -- fulfill your pledge "ka'asher nadarta," with the same intensity of feeling as when you first made it.
Klal Yisrael wanted to capture the ecstasy of that moment of shirah so it would last forever, so that every time in the future they would sing shirah it would be done the same way. Va'yomru, they sang the shirah, leimor with such exuberance that it would be said that way again and again for eternity.
Maybe that's in fact why it's called a shirah. When a song becomes a hit people listen to it again and again, it seems to never grow stale. We're still singing the song, hopefully with the same enthusiasm.