I absolutely love making things out of cardboard, it's amazing what you can do with it. I'm no expert on it, but I'm still amazed at the things I can do. Here's some tips that may help people who are just starting out.
2. Cutting Tools
Black & Decker cardboard cutters. They're not very great for cutting out small things, but they're excellent for cutting out large pieces quickly and rounding edges. There's also virtually no way to accidentally cut yourself with them! The drawback is that they take a long time to charge and then don't hold a charge for very long. Also, you can't use them while they're charging.
3. Clean as you Go
This is so simple, yet so difficult. Keep your working area free from scrap pieces, and everything will go a lot smoother. Put cardboard that you haven't cut yet off to the side.
4. Paper Mache
Cardboard can be used as a skeleton for paper mache. I'll make a tutorial on this later (even though I suck at paper mache), but basically, you make the shape you want out of a flat piece of cardboard, then cut smaller pieces or cardboard in the width that you want the project to be. You cut half-way through both pieces and put them together perpendicular to each-other. It'll make a lightweight, sturdy skeleton that you can then cover with paper mache. (Also, if you're making a large prop like a staff, you can put air-drying clay on the bottom of the cardboard before you apply the paper mache so that it's more sturdy.) Like I said, I'll make a tutorial some other time, but it's something to look into if you're interested.