There's a surefire way to get your kids to do something - tell them they can't. We laugh about this sometimes, but it's a truth we, as parents, ought to heed. 

There's a story about a study being done in an elementary school. The researchers planted some flowers on the playground and let them grow. All school year the flowers bloomed and the kids barely noticed them. Then one day near the end of the year, one researcher came into the class and said, "Now kids, see those flowers out there? Whatever you do, don't spit on them. We want them to keep growing and it's really important that you don't spit on them." They secretly video-taped as over the next few weeks every single kid went up to the flowers and spit on them!

It's really easy to just set up the rules and expect your kids to obey, and if you're tough enough you might get them to fear you enough to do as you say, but there's a better way.