There are several Good ways to think about wrist posture.One way is to keep the hand in the same plane as the forearm,as this person is doing here by using a wrist rest along with the computer mouse.
a slightly more accurate approach is to keep your hands more or less like they would be when you hold the steering wheel of your car at the 10 and 2 o'clock position - slightly in and slightly forward.
Here's an example of how this principle applies to tool design.Working continuously with the pliers as shown in the left-hand picture can create a lot of stress on the wrist.By using pliers with an angled grip.however,the wrist stays in its neutral posture.