I found around kids that it helped tremendously to keep the rules few and simple... and then to be CONSISTENT in enforcing them. Every parent will get challenged now and again, but the parents who get challenged by their kids the most often are the ones who are the least consistent and don't make the rules clear.... (sometimes "no" means mommy will give in if i keep it up....)
I keep thinking about the whining we all see in the stores... some kids whine, beg, tantrum, etc, because they know they can sometimes get mommy or daddy to give in and they've got little to lose by trying. It then becomes their extortion... to wear mommy and daddy down or embarrass them enough that they give in, or to appear so miserable that mommy or daddy feels the hero at making them happy with candy. But it doesn't make them happy in the long run, because they learn how to be discontent whenever they don't get their way and their candy or silly toy and absolutely any impulse they have. What a sorry lesson to teach them for life! Kids with parents who don't give in to extortion have happier kids and they'll be happier adults.... they learn how to be happy themselves without a whatsit or getting others to give in unhappily. Tell the kids before the shopping trip whether or not they will get a treat this time... and then don't give in in the heat of the moment. If they ask for something say no ONCE... and ONCE ONLY... if they ask again, or whenever they ask in a whine or demand don't respond at all! Act like you're completely DEAF and they'll soon learn they have to ask nicely to have any chance at all and that no really means no.
When i babysat many different kids, i always made the rules clear, the kids knew they would get ONE warning and one answer, and they knew if they didn't listen to the warning the punishment would be a time out (or i'd quit playing, or whatever, depending on their age and the situation). I tried always to give a warning... this is how they learned to mind my word. In a sense, they learned they had control in this way... they could choose what would happen... their world seemed a lot more secure to them, too, because knowing the rules and how you will respond makes it so. Sometimes i've seen parents punish without giving a warning to stop... this is terrible. The child only learns then that punishment is unavoidable, and their world isn't secure. I wouldn't yell at the kids (unless they were running into the street!), we wouldn't get into any struggles or arguments, they didn't feel like i disapproved of them ever... only bad or dangerous behavior. It established their strong trust in me. It worked really well for their parents and the kids and me, and we were all very happy with it and had a good time together.