These days, there's a major trend in the fitness world when it comes to heart rate monitoring: optical heart rate sensors in wrist-worn devices like activity trackers or watches. Monitoring HR from the wrist makes a lot of sense, especially if you're already going to wear a watch or fitness bracelet. It sure beats strapping something around your chest (and making sure the electrodes are making proper contact with your skin).
As I've played around with different wrist-worn gadgets that include optical HR sensing, I've noticed that some are much more accurate than others. I've been very impressed by the HR accuracy on devices that use a Mio HR sensor. I've been disappointed by the HR accuracy on Samsung fitness watches and bands. So, if you're serious about training by HR data, make sure to get a device that will give you accurate information. In general, chest-worn HR sensors are very accurate as long as the electrodes have good contact with your skin. You can improve that contact by using electrode gel or water.
If your wrist-worn HR sensor isn't accurate, there may not be much you can do to improve that accuracy. You may be able to adjust the band or strap to reduce the amount of ambient light that hits your sensor. Maybe you can make your own "do it yourself" type of seal that attaches to the back of the watch to reduce ambient light. I think I'll make that a summer project and I'll try some foam tape or other materials to create an "ambient light blocking seal."