I have a "quick and dirty" method which works pretty well.

However, the real secret, no matter which method you choose, is not to try to cut all the way through the tube in one pass. Instead, rotate the tube holding the blade steady, applying a light pressure to only cut a little way into the tube. As you rotate the tube several times, you cut deeper and deeper until it is cut all the way. I have had very good luck following this pattern.

My "quick and dirty" method is to draw a line around the tube where I want the cut, and then wrap several layers of tape around the tube on the "good" side (the side that will be used) of the line. I use drafting tape since it doesn't stick as much and is less likely to damage the tube when being removed, but masking tape will work. This gives you a shoulder to help guide your blade and keeps you from accidentally cutting into the portion of the tube you will be using. When removing the tape be sure to pull the last layer off the opposite direction from the spirals so you don't delaminate the tubing.

From a post by JS in rec.models.rockets