Introduce manners to you child early on, preferably as soon as he begins to talk. The younger the child is when he begins practicing manners, the faster he will incorporate them into his behavior.
Model the manners you want to see in your preschooler in your own behavior. Your child is much more likely to say "please" and "thank you" if he sees you behaving--and speaking--graciously to your spouse, friends, co-workers, other parents and, of course, your children. Don't make the mistake of being super polite to strangers and rude to your own family members, as preschoolers will pick up on this discrepancy.
Introduce one rule of good manners at a time to avoid overwhelming your preschooler with too many new rules to follow. If you want to concentrate on table manners first, for example, start out by showing him the correct way to hold a fork. Move on to other tasks, like keeping your elbows off the table or placing a napkin in your lap, only after he has mastered the first skill. It may seem like a slow process, but choosing one skill at a time helps kids feel confident in their abilities, instead of frustrated and overwhelmed.
Praise your child for his successes as he practices good manners, instead of punishing mistakes. Positive reinforcement helps your preschooler feel good about learning good manners. No child, at this age, is going to use good manners all the time, but receiving regular, positive feedback will make him want to try even harder. Tell him how pleased you are that he remembered to thank Grandma for the gift, for example, but try not to constantly nag or pick on a preschooler for a good manners skill he is having trouble mastering.
Display a copy of the Golden Rule in a central location in your household, and make sure your preschooler knows what it says. Teach him to treat others as he wants to be treated. This will help him understand why we use good manners--they are an important way to show respect and kindness to others. Your preschooler recognizes how good he feels when others thank him or apologize to him when necessary, so use these events as a discussion point when teaching good manners to your child.
Celebrate your preschooler's good manners by throwing a special dinner party just for your family or for his preschool class. Set a table with fancy glasses, plates and a tablecloth, and invite attendees to wear their finest attire as they eat, drink and converse, while using their best manners. Having a special event like this to look forward to will help your preschooler focus on practicing good manners on a regular basis, as he anticipates a special treat.