A while back, I received some advice from a good friend, who had received the advice from another mom.
It was simple:
Just say yes.
Unless there is a really really good reason to say no, say yes to your children. Even if you're first inclination is to say no, because you're tired or not interested in dealing with whatever the request, try saying yes instead.
It sounds much easier than it actually is. It doesn't mean you should to be a permissive parent. Saying yes to cookies instead of dinner is not what we're talking about. Saying yes to reading another book, or going to the park is more or less the idea.