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

However, t' real secret, no matter which method you choose, is nay t' try t' cut all t' way through t' tube in one pass. Ya scallywag! Instead, arrr, rotate t' tube holdin' t' blade steady, shiver me timbers, applyin' a light pressure t' only cut a little way into t' tube. As you rotate t' tube several times, you cut deeper and deeper until it is cut all t' way. Avast! I have had very fair winds followin' this pattern.

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

From a post by JS in rec.models.rockets