We have a Snuza monitor. It attaches to the diaper and senses the movement of the belly. I love it because it doesn't matter if he is sleeping with me or on his own, I don't have to move it. And if baby moves around in their crib it still senses them.
I had a couple false alarms because I switched diapers. They were very thin and the alarm didn't stay snug. I can either fold the top of the diaper in or use different diapers. I like the PC Green which are biodegradable and they are hold them nicely. The Huggies Natural I have to fold. But either way, the monitor works!