Hobby Knives - Cut cut cut.
Picking the right hobby knife for papercraft is crucial for getting precise and clean lines, and making the work comfortable for you. The exact brand doesn't matter that much, any hobby knife will do, but you'll want to get the right kind of blades for your knife. You'll want to get blades that have a pointed tip that is at a fairly sharp angle. This lets you cut into the really sharp corners that many papercraft pieces require. The good ol' X-acto knife is what I prefer for all my hobby work, and if you go with X-acto, the type of blades you'll want are the #11 blades. These are made for precision work, and are perfect for cutting paper.
You'll have to find a knife that you're comfortable with using. Should the handle be thick or thin? Should it be metal or rubber? These aspects will depend on what you're most comfortable with, and how much precision you can get from it. When I'm doing long sessions of paper cutting, I like knives with a comfortable rubber coating. When I need to cut very intricate pieces, I like skinnier metal handles to give me the precision I need.