I cannot tell you how much it annoys me when people smack on their food. I understand that it is considered good manners in some countries, but my home is not one of those countries. I do NOT want to hear or see someone's food as they are munching it down. I am happy to see that there is someone else out there who agrees with me. [link]

WikiHow: How to Chew with Your Mouth Closed

Chewing with your mouth closed is good ettiquette as you grow up. Sometimes, it is hard to remember to chew with your mouth closed. You might also need some practice.

Steps
1) Practice with chewing gum. Close your mouth and chew with your back teeth. Use one or two sticks of gum for this practice. Remember to breathe through your nose, not through your mouth.
2) Start out with small portions of food; first soft food, like ice-cream or yogurt. Put a little bit of the food into your mouth. Now close your mouth, chewing with only your back teeth. Chew slowly; this helps you keep your mouth closed.
3) Once you have mastered it with the soft foods, try chewing larger portions at a time, as you would normally eat. This is no reason to open your mouth. Still chew with it closed.
4) Move on to the more solid foods now, like fruits, rice, and bread. Follow the same principle: small portions at first, then slowly worrk your way up. Chew slowly.
5) Once you are past this stage, try the hardest foods, such as granola bars and cereals. While these may make noise, it is no excuse to open your mouth while chewing

Tips
Reward yourself for every day you chew with your mouth closed. It will help and encourage you to continue improving.
Ask someone to monitor you. Believe me, they know about your poor manners and would be happy to help you improve.
If you are having trouble believing you look awful chewing with your mouth open...set up a video camera or mirror and check yourself out.
If you find your self eating alone, print this out and read it prior to eating your meals.