Wear a cotton shirt or thoroughly wet both sides of the strap.
Synthetic fabrics that rub or flap against the heart rate monitor can create static electricity that interferes with heart rate signals.
Move away from sources that can interfere with your heart rate monitor.
Sources of interference may include strong electromagnetic fields, some 2.4 GHz wireless sensors, high-voltage power lines, electric motors, ovens, microwave ovens, 2.4 GHz cordless phones, and wireless LAN access points.