Developing effective communications patterns with kids

How can we teach our children good listening skills? They seem to have trouble paying attention to instructions and following directions. In my daughter's case, I think she's just defiant most of the time. As for my son, he's very intense and tends to get so locked into television, a book, a videogame, or whatever he's doing that it's almost impossible to pull him away. I find that I'm repeating myself over and over again. Do you have any advice?