A tubing cutter will work. However, you need to back up the tubing with something like a wooden doll rod so the tubing doesn't colapse.
I have also used a rigid plastic pipe cutter...
See post # 198 in the following
thread.
http://www.e-cigarette-forum.com/fo...-tank-mod-map-tank-gp-ssr-20.html#post3363171
Using a rigid plastic pipe cutter isn't the greatest way to cut it. However, it works.
Edit to add;
I put some blue painters tape over the tubing, mark it, and make a couple of passes over the tubing with a tubing cutter and then cut it on my band saw.
Once you get that nice straight starter cut made with the tubing cutter you can even cut it by hand using a small coping saw, hobby saw or any kind of a fine tooth saw.