Personally I think there is nothing better than the Exacto blade with #11 blades you find at any drafting supply store. Use it was a metal cork back ruler to cut sstraightlines. (the cork protects the work surface - you should place the ruler over the side you want to use to protect it and cut with the blade twisted slightly out so if you mess up it doesn't cut what you want to use) Also is great for freehand work. They make a swivel version if you like cutting lettering from card stock. Watch your fingers though! These babies are sharp.
Draw lines in reverse on back side of the paper you want to use for guides in this type of cutting.
If your cutting becomes hard to do or isn't producing a clean edge, change blades. Be sure to safely dispose of old blades by putting them in a special container - like an old plastic container with a slit cut into it for blade insertion.