As Powder mentioned lots of folks have had fine luck using steel hose clamps to gently squeeze the tube while cutting with a single edge razor blade or Shape X-acto blade. Key is the blade should be NEW, or stropped razor sharp.
It's also advisable if trying to cut tubing without an internal mandrel to try to do the cutting of the tube on the "PULL OUT" part of the blade stroke. This helps reduce the amount of damage to the interior of the tube.
I guess the most important "tip" I can pass along is to get really clean cuts on most Standard thickness and heavywalled Body tubes is to use an internal mandrel to backup the area under the knife blade. This is how the tube manufacturers do it. They have hard rubber expandable roller mandrels, We have to make do with spent motor casings, wooden dowels, PVC pipe, Heavy cardboard tube cores and built-up areas on these items with masking or Duct tape.
Driving a 1/2" down thru 13mm motor casings, 5/8" in 18mm and I believe 7/8" for 24mm casings you can sand and CA the exterors to make some great, very long lasting mandrels.
Once you have a set of internal mandels made they'll be around for a very long time.
Years ago I built an all Scrap aluminum cutting jig that works great but seems to been a little over the top for most folks
The reason I chose aluminum angles to build this thing are First; The pieces involved are truely scrap I had laying around the shop. Second: Aluminum is stable, not tending to warp over time as wood parts have a tendency to do.
Others have used wooden corner angles or nailed or screw joined 1by angles for their jigs with fine results. The big thing is to have a way to adjust the depth of blade being used to cut into the tubes and some sort of adjustable stop to allow repeatable cuts. This can be as easy as an additional block of wood and a C-clamp. Again I'll stress the need for internal mandrels to really get factory quality, no sanding required tube cuts. I use my little 19" jig to cut all size tubing from under 1/8" to BT-101's (about 4").
Hope these pics will helps.