Here is a list of tips and tricks on how to improve the accuracy of the heart rate read from the Apple Watch.
1. Optimize skin contact
First and foremost, the sensors need direct skin contact so make sure you wear it under your shirt with the watch on top of your arm. The recommended fit is snug but comfortable.
2. Blood flow affects accuracy.
It's important to note that skin perfusion is different for everyone and that the quality of your skin’s blood flow affects the accuracy of the Apple Watch's readings. For some people the sensor is prone to inaccuracies.
3. Avoid cold temperature.
The cold will reduce your blood flow, consequently reducing the accuracy of the heart rate sensor.
4. Shy away from irregular movements.
The less your wrist moves the more accurate the readings will be. If you know your workout involves a lot of wrist movement tighten the strap accordingly.
5. Sync to an external heart rate monitor.
If you are not happy with the accuracy of the Apple Watch optical heart rate monitor you can always connect your watch to an external Bluetooth heart rate monitor for more precise readings.
NOTE: A Bluetooth heart monitor is only able to connect to one (1) host device (Phone, Tablet, etc) at a time. If you've previously paired your heart monitor with another device, it might automatically be connecting to the other device. A trick to prevent the other devices from stealing your heart monitor's connection is to turn off the Bluetooth radio from all other nearby host devices.
For more information on the subject you can check the following articles:
If you require even more precision, you can pair an external Bluetooth Heart Rate Monitor to your Apple watch.