Wrist strains and sprains are common amongst boxers. Regardless of your level of skill, a sore wrist should start by evaluating your technique, particularly the position of your fist at the point of contact. Some wrist injuries come from chronically throwing punches incorrectly and others can be simply overuse. One of the most common causes of wrist pain in boxing is from throwing the hook punch. Although wrist injuries can be mitigated with additional wrapping or taping, I prefer to see everyone learn to throw the punch properly instead.
To avoid injuries to the wrist focus on the hook punch; keep the elbow at the height of the wrist or higher. Make sure you avoid 'breaking' the wrist alignment when throwing all punches. I see this mis-alignment occur in the hook punch and uppercut more than straight punches.
Although there are many other tips to throwing the hook correctly, this is one tip to specifically avoid wrist injuries. This tip applies to hitting mitts and the bag.