Manners are important to me . . . and to my husband . . . and maybe even my 13 yr. old, but not so much to my younger children (although they're not terrible). I know it's OUR job as parents to help our younger children understand why manners are important and to teach them proper behaviors. And what better place to practice than at home?! :)
my biggest offenders
We have a lot of friends over . . . I'm not awesome at planning playdates, we are just lucky to have lots of kiddos close enough to ride bikes or walk over when they want to play. For the most part they are sweet kids, but sometimes lack manners. I'm not trying to imply that my kids have perfect manners -- they don't. But I'm used to them, and their behavior - so when I watch their friends, I can see more clearly what I want my kids to do and not to do. (Does that make sense?)
For instance,
I don't want my kids raiding other people's pantrys or fridges without an ivitation.
I don't want my kids being rude or mean to their friends little brothers and sisters.
I don't want my kids telling adults or older siblings that they 'are not the boss' of them.
I don't want my kids to be disrespectful to others - friends and parents alike.
I don't want my kids to be neglectful with other people's property (like standing on the furniture, or leaving tools or bikes outside to get rained on, or putting holes in the walls, or breaking lights, or toys, etc. -- BOYS!).
And, I don't want my kids to walk into someone else's house and not address the adult(s) that live there with a kind greeting when one is offered to them.
I DO want my kids to say "thank you for having me over".
I DO want my kids to help clean up after themselves when they are at someone else's house (and their own house for that matter!).
And I DO want my kids to do the opposite of all the afore mentioned shananigans. :)

So, I've thought about it, and here is a list of some manners I hope to instill in my children:
1. table manners ~ basic etiquette . . . could be it's own post
2. introductions / greetings ~ Don't call adults by first name only, smile, look in eyes, speak clearly, etc.
3. please - thank you - sorry - excuse me ~ common sense manners, a little effort goes a long way
4. how to take compliments ~ be gracious (don't say 'I know' when someone says you have pretty eyes - and don't say 'no I don't' when someone tells you that you look nice. Just say thank you :)
5. holding doors - the rules of entering and exiting ~ always let exit-ers exit before you enter, hold door for others
6. telephone manners ~ how to answer the phone, how to take a message
7. not interupting other people when they are speaking ~ ehem . . . like when mom is on the phone
8. good sportsmanship ~ how to be a gracious winner and a gracious loser
9. respect differences ~ always be kind (don't tease)
10. clean up after self ~ no matter where you are
Visit raising lemons for teaching kids all sorts of manners.
My husband grew up in the south, where manners were emphasized. For example, he was required to say "yes, ma'am", and "no, sir". When addressing adults he NEVER called them by their first name unless he put a Miss or Mr. in front of it (like Miss Kjirsten, or Mr. Quinn). And formal dinners were common place.
These were three things that I fell in love with when I married into this Southern family. And I have worked my darndest to continue these traditions in my own family. It is certainly a work in progress . . . but certainly worth the effort!!!!
My husband tells our children this, all the time. Good manners can go a long way! ;)