Obviously I spend a lot of time painting paper, but you might be surprised to know how much time I spend cutting paper. Rarely do I finish a watercolor on an appropriate sized sheet of paper. Most of the time I paint on large sheets that then have to be trimmed down once the painting is finished. Sometimes the leftover piece is big enough for me to paint something else on, sometimes not.
I also have to trim every bookmark and magnet insert, print, and invitation that I make. I use a large format printer and print everything (except the note cards) on 13" x 19" paper. Then, I have to cut whatever I printed down to the appropriate size.
Enter x-acto knife.
Every artist has one and they're quite useful for a number of things. I have several floating around, but lately all I've been able to put my hands on is the one I like the least.
Anyway, it still gets the job done. The only other thing you need is a straight edge, like a metal ruler (another artist staple). I also have a cutting mat, but a piece of cardboard would work too (a cutting mat will keep the blade sharper for longer). Just firmly hold down your straight edge and then drag the x-acto knife alongside it and... voila! You'll get a super clean, super straight line. It's a lot quicker than using scissors and produces a straighter, more uniform edge.
I also have this nifty contraption that my brother gave me a couple of Christmases ago.
It's great for cutting large volumes of stuff, like a whole slew of bookmark inserts. It's not quite big enough to cut 13" x 19" paper though so I have to half the paper first with my trusty x-acto knife, and even though you can cut several sheets of paper at a time, I only cut one at a time. It's too easy for the sheet(s) underneath to get off track and then I end up with a whole stack of stuff I disfigured.
Speaking of disfigurement... I did slice a decent sized chunk off my left pointer finger with an x-acto knife a couple of years ago but since then, so far so good. That once incident pretty much taught me an awareness that I don't think I'll soon forget!