Please and thank you - start early
The general consensus is that it's never too early to teach your child how to say please and thank you, with some suggesting that kids should be taught to use good manners as soon as they're able to understand you and verbally communicate. When your baby is around 18 months of age onward, then, you can start teaching them the basic principles of good manners.
Lead by example
According to etiquette experts Rudebusters.com, "Actions really do speak louder than words" -- so if it's important to you that your kids have good manners, then you need to lead by example. "Some studies show that children can show signs of empathy and concern from a very early age," the website advises. "In other words, parents have the power to nurture, guide, show and instill."
Kids are complete sponges: they will watch you and imitate your every move, so work this to your advantage. You can't expect your toddler to be polite and gracious if you don't display those qualities yourself -- after all, having good manners is a learned behaviour, and you are the teacher.
Begin with easy manners such as "please" and "thank you", or their much simpler alter ego: "ta". FamilyEducation.com suggests that you phrase these as the "magic words", as it will feed into your child's already-strong urge to learn and master new words. These can be taught very early on, but it will take time for your child to learn, so be patient.
As your children get older, teach them how to use cutlery instead of their fingers when eating, and enforce a rule whereby they need to ask to be excused from the table during meal time. Show your child how to shake hands firmly and address adults using "Mr" or "Mrs" where appropriate, and explain to them why it's important to be respectful to their elders.
Praise good behaviour
Positive reinforcement works wonders, so make sure you reward good manners when your children do the right thing. That doesn't mean you have to gush -- just let them know that their behaviour was noticed and appreciated.
Jump on problems immediately
When your kids are toddlers you can't expect miracles, but as each year passes you can afford to become stricter when enforcing good manners. If you're in a situation where you expect your child to say please or thank you and they don't, address the issue then and there.
The most important thing you can do is demonstrate to your child that their actions have consequences. This applies not only when you're teaching your kids good manners, but in all aspects of life! It means that you need to follow through -- so if you see them display disrespectful or rude behaviour, let them know that it's not acceptable.