The last clause of your last sentence is key. Just saying "no" isn't always enough to get results. I was constantly on my kids to sit up straight, take your hat off when inside, feet on the floor, open doors for other people, dress for dinner, etc., and I tried to explain "why" even if the explanation was as silly as "I don't want you to make a fool out of yourself if the President of the United States ever invites you to the White House for dinner." The other thing that was key was taking action swiftly and consistantly. I can recall being at a fast food restaurant and my son who was five at the time wanted to get out of his seat and run around the dining area like the other kids. I gave him one warning. When he disobeyed me, I picked him up and we left without his happy meal. He screamed all the way home as I explained why we had left the restaurant so abruptly. By the time we arrived home, he had calmed down and wanted lunch. "Sorry Charlie, but you already brought you lunch and because of your bad behavior we left your lunch on the table at McD's. The next meal for you is dinner." I never had to tell him to sit in his seat again at a restaurant. When I told my sister what happened, she said I had been unreasonable because all kids run around at McD's. I told her that a five year old can't discriminate between McD's, the diner, or more fancy restaurants and if you let them run around McD's they will also do it at fancy steak house. I also told her that I don't have to be reasonable to a five year old.
Jesus H. Christ, I wish there were more parents out there like you.
And I thought ours was the only house to use that reason at the dinner table!