L., I put my first child on his side until he was older and wanted to be on his back. (That was when I moved him to the big crib from the bassinet.) However, my younger son couldn't settle on his side or back. He was miserable. I talked to the ped about it and she told me to try his stomach. She explained that some babies just don't sleep well unless they are on their stomachs. She also reminded me that millions of people lived after sleeping on their stomachs before doctors said not to do it. She is right. I put him on his stomach and it was like magic.
What is important is to go by the rest of the SIDS guidelines. No toys, pillows, etc in the crib. A sleep sack instead of blanket is a good idea. Don't have the room too warm either.
Dawn