Charcoal is an absolutely magic medium for your drawings. Especially when you are doing a portrait, charcoal makes it both soft and realistic at the same time. Its dark rich color and gentle fragile texture add deepness and intense look to your portrait. Nowadays there are a few popular types of charcoal for drawing: vine charcoal, compressed charcoal sticks and compressed charcoal pencils. They are all great and almost the same in the process of drawing. Charcoal, due to its brittle nature, is easy to smudge which is not always a good thing. Slight wipes that help you to make a perfect shading and a smooth fading can also lead to smudges, if not careful. You should remember that charcoal is messier and dirtier than graphite or other mediums. That is why you can use My Tip #5.
When you are working on details always keep a piece of paper under your hand. This way your drawing will always stay untouchable and your hands clean. Another rule that you should remember is never wipe the charcoal off the paper with your hands, just blow it off or use a kneaded eraser. At times charcoal tends to leave spots on paper that are almost impossible to get rid of sometimes. At first it may seem like not a big deal, but even one little imperfection on the background can ruin your portrait.
I hope it helps you to reduce mistakes and keep your portrait perfect.