I have a Polar HRM so I'm not sure if my tricks will help, but I had better luck when I moved the strap lower down on my ribcage--I think the contact/rubbing with my bra strap would make the sensors lose contact with my skin, so now I put the strap about an inch below the bra. After you get over the sensation that it's about to slip down (it never has) it's fine. I also use a store brand version of K-Y jelly on the contacts, particularly in cold weather where I can't count on sweat to keep the contacts wet. I've noticed a hierarchy of effectiveness--the HRM works most consistently when I'm swimming and least consistently when I'm cycling, which makes sense--when I'm swimming it's wet all the time and I have no bra on and I"m wearing a skin-tight suit, and when I'm cycling the constant breeze dries off my skin pretty quickly and my position makes the strap lose contact easily.