1) ALWAYS have your DH/Partner back you up 100%. In fact he has to start by telling the kids "Things are changing. I am not proud of the way you behave and if your SM/*Your name* tells you to do something you will do it. No questions."

2) Insist on hats off, clean hands and face and shirt on at the dinner table. Not negotialbe unless the hat thing is a religous item.

3) NEVER let a child push past you out the door unless there is an emergency like a fire or gas leak. You grab them by the shirt collar and pull them back. You get down to eye level and say "Adults first. Do NOT forget it. One day you will be an adult and no snot nosed brat will be allowed to push past YOU either."
The next time they push past you grab the shirt collar and say "Taxpayers first. Children afterwards."
The 3rd time it happens you say in a loud voice "Excuse me? Are you unable to remember anything? Who are the adults and tax payers here?"
Partner/DH has to back you up 100%.

The only time they are allowed first is if they are holding the door open for adults.

4) Fighting in public or bad manners and you get The Stare. Do it again and we go home. I don't care if we are in a movie line or in the movie. We go home. Waste of money? Yup. But the dorks-in-training miss out on the treat. And then as you drive home you remind them fighting or bad manners in public or at home is NOT tolerated. I would even pull them out of a sports game if they were playing up. In the car and home.

5) Burping/belching/farting/breaking wind in ANY company is excused with a "pardon/pardon me/ excuse me". DH/Partner needs to be 100% on board. Even if everyone laughs about it, common courtesy says to excuse yourself first.

6) Use hand signals. If the D.I.T. interrupts your conversation all the time hold up your hand and don't look and the child. If he/she keeps talking turn with your Steely Gaze (watch Super Nanny. You may need to invest in a cheap pair of glasses but I am lucky because I have an accent that is mistaken for her all the time. I can silence a child with the Steely Gaze in a second but you can do it too). Look over the top of you glasses and drop your voice 2 octaves (esp if you have a higher pitched voice) and say "Excuse ME! I am speaking and unless there is blood, broken bones or fire or someone has stopped breathing I am not interested. Wait your turn." Continue your conversation and when you have finished your immediate topic turn to the child and say "Thank you for waiting. Now what did you want to tell me?"

7) Most important words... Please, thank you, excuse me. My daughter (now 28) was an average student. No honour Roll or class awards. Just a plodder. BUT she shone because she had good manners. These good manners got her her first job. These good manners has her with the respect of her peers. It works that way. But we need to use these words as well.

Always aknowledge good behaviour with a smile and a thank you. And if you observe good behaiour and catch the childs eye, smile and wink at them.
Always be firm but fair. Don't make kids wiat for you forever and be consistent.
And no child should be rewarded for holding a door open for an adult by getting icecream or a treat. No treats for good manners.

Children are a product of their environment and unless you have the 100% backing of your DH and he is also on the School Board of Acceptable Behaviour.

We are expected to break a child of xx years of slovenly manners in a few months? It cannot be done unless the Birth Parent recognises the disservice he/she is perpetuating on their child/ren and agrees to the Plan of Attack.

By the way, the second you are undermined you disengage entirely. No school runs, no grocery shopping. I am firm but fair. You undermine ME and I am the room mate. And yes, it even comes down to sex. You undermine me, no sex. I did it one time only. I refused to sleep with him unless he manned up and stopped allowing his kids to be an embarrassment when we were out and about. He found out that I was not about to lower my standards (which are not excessive)to accomodate his guilty parenting.

Good Luck!