Since most heart rate monitors use electrodes to measure your heart rate, it's important to ensure optimal conductivity.
Firstly, the electrodes need skin contact in order to work, therefore the monitor is worn under your shirt against your skin. It's also recommended to wear the strap on the ribcage, below the pectoral muscles or breasts.
Secondly, it's important to properly moisten the electrode bands. Sweat will take care of this once you're in your workout, but you might want to apply saliva on the bands by licking your fingers and spreading it onto the electrodes. You can also apply heart rate gel (available at local running stores or online).
Finally, ensuring that you have sufficient battery power is important otherwise you might have false and sporadic heart rate readings.
For more information here is a link to a great article discussing all aspects of troubleshooting a bluetooth heart rate monitor and eliminating spikes and dropouts.