As much as I love to switch out the music on our ipods almost weekly (well before the baby came and we ran out of time to even use them much) I don't buy mp3s that often anymore. Part of it is just because the cost ads up so quickly if you aren't buying those mp3s on sale. That's why I love that Amazon offers free mp3 downloads of certain songs.
To find the free song download of the day just visit the MP3 Specials page. In the left hand column you will find the free song of the day. You can add it to your cart and check out as normal, but the song price will be free. The right hand side of the page will also give you a list of the top free songs and albums!