Using the right tool for the job makes a huge difference, no matter what that job is. To most people, folding paper probably does not seem like an act that would require a special tool. You just fold the paper in half and run your finger along the fold to make the crease, right?
When you use a bone folder, that crease it tight, even, and clean, and it stays folded. It turns a task you never really think about into a pleasure. When you can turn the ignored corners of your life into enjoyable acts, then your life is that much better!
You can find bone folders at most art supply and all book binding supply stores. Unfortunately, it has to be bone. There are plastic folders out there, but they're just don't work as good as the real bone does. We have several in our house, and we keep them in those little nooks where the pens and pencils accumulate, because you never know when you're going to need to fold something. When you do, it's good to do it right!