Whether you make your if/then statements in the negative or positive has a compounded effect over time. I think it is easy for us to forget just how much kids have to put up with adults setting limits for them. And don't get me wrong, I'm all for adults setting reasonable limits for kids. Kids need them. But lets pause and consider (or even remember) what that's like. They have to listen to parents, teacher and other adults throughout their day telling them when they can eat, that they have to be quiet, that they can't play yet, that it's time to clean up now, and on and on.
The point is that when you add up all these commands, the positive manner of phrasing them leaves your child focused on the options at hand. They sound more like the world is filled with opportunities, and they have choices to make. It helps them to behave more responsibly and feel more capable: "Feel free to dig into those cookies, as soon as your lunch is finished." On the other hand, when stated in the negative, the child ends up with what must feel like an avalanche of threats, constraints and negativity. "If you don't get that desk clean, you're not going to recess", "If you don't finish your dinner, no chocolate milk for you."
The negative command tick is a challenging habit to break. See my article at GTDtimes about elephant training to understand why that is, AND what you can do to make your success much more likely. Also see this previous post on seven tips for mastering new interpersonal skills.