I hate these packages. I really do. Usually I'm stuck using the knife I have on me when I buy something. I use the tip to get into the package and carefully slide the rest of the blade under the plastic, then cut around the item inside.
Or, if it's especially tough plastic, I use the part of the blade closest to the handle for more control.
Really though, heavy scissors really are a much better idea. If you really do need a pocket knife to cut these open, I use either a very small bladed folder or fixed, or one of the many folding boxcutters that are available that only have a small part of the razor blade exposed.