It always helps to study the works of the masters. If you go through the galleries, pick the ones that look the best to you and try to tell what it is that makes them realistic. Is it a realistic expression? The way the shadows fall?

Then compare yours to that - do my surfaces show the same properties? If not, what do I need to change to get that effect? How do I make the shadows in my renders look the same? How do I get the same soft lighting?

Technical skill is nice, but it only takes you as far as your eyes see - just like drawing or painting, digital arts require you to see the things before you create them.