Rewards need not be extravagant, just meaningful. Sometimes, of course, you can't avoid trouble. Almost all children, sooner or later, will cross the line you've drawn. It's their way of learning about trust or finding out if you'll really do what you say. Resist the urge to come to to rescue of the child.
Children learn best when they learn for themselves.
Logical consequences . Natural consequences work best, but they are not always appropriate. If your children don't pick up their toys, they'll be in their way. But that will bother you more than them. So you need something more.