These are three of the most common mistakes I see in beginners work.

1). Get the shadows right. You don't have to leave the Shadow intensity of a light at its default value. You may need to touch-up the shadows in postwork.

2). Get the eyes right. rarely should they look straight ahead. Think of dilating or contracting the pupils. Look at the eyelids. Pay particular attention to highlights in the eyes.

3). If the elbows, knees, or thighs, are bent to any degree, they will probably need postwork.