There are a number of ways that you can prevent crumbly bars. First, use natural peanut butter rather than regular peanut butter for the best consistency and to ensure that the cereal mixture adheres together. Secondly, it's important that you line the pan with parchment paper to make it easier to lift the bars out of the pan and cut into bars. If you try to cut the bars right in the pan, they'll be much more crumbly when you try to remove them. Finally, you'll want to make sure that the cereal mixture is pressed down very firmly in the pan. If it's only packed loosely, the bars will be crumbly when you try to cut them.