Be encouraged! You have some good answers already!
When you're out with your children, ask them to look around, QUIETLY, for the people who have pleasant table manners, who open doors for others, who help when help is needed, who (gasp!) stand up when an older woman enters the room, and report these things QUIETLY back to you. (When you have to talk about other people's good points, the well-mannered way is to do it very quietly, without attracting attention.)
Here are just a few thoughts: Etiquette is nothing more than a standardized method of remembering to be kind to other people. It's not stiff and stuffy. One cannot learn good manners from TV or movies. You have to look at real life. It's selfish people who don't wait their turn in line. A person with bad table manners has no conception of what his/her tablemates are having to put up with. If you really think about the other people in the room with you, you won't want to interrupt their conversation... unless the house is on fire. People who barge along on the sidewalk or in the aisle aren't even thinking about the stranger walking toward them. For youngsters to respect older people is not a sign of inferiority but a sign of strength.
Next time you're at the library, find Betty McDonald's MRS. PIGGLE-WIGGLE'S MAGIC, and read the chapter called "The Bad-Table-Manners Cure." It'll make you feel better! There are some other good-manners stories in her four books, too.
You and your husband, of course, are modeling the manners you want your kids to have, and they really ARE observing you. Even when you wish they weren't.
You might point out that grownups are more likely to look around - side to side - than they are to look down, and may not notice a boy or girl in their path. So it's up to the kids to notice the big people first and make sure they don't get startled or fall. What are other reasons for doing mannerly things? If you can emphasize the good reasons, your children might remember the instruction a little better (although they might not show it to you for a while).
When they start to be interested in the opposite sex, you might point out that good manners are something the opposite sex REALLY notices. The guys who forkload food and shovel it into their mouths at the school lunchroom don't like the girls to be doing that. The girls may tolerate ill-mannered boys but don't think much of them.
So don't throw in the towel! You may not see the result until they're adults! :^)