I kept coming across this thread in my own quest for a better way to cut heatshrink, so I thought I should post what I eventually came up with. I found a tool called The Chopper II, which model railroad enthusiasts use for... whatever it is that they do. It's a bit spendy ($40 to $50) as compared to scissors, but it works beautifully. It's basically a little cutting table with a craft razor blade on a pivot arm. It has gridlines on it, and adjustible stops (one of which is 90 degrees), which makes a nice straight cut of a precise length of heatshrink fast and easy. I often have to cut a few hundred pieces at a time to a particular length, and for that purpose it's invaluable.
I wrote an article for Make magazine, and on my own blog that goes into more details. However, a careful reading of the posting guidelines suggests I am not allowed to link to it. Mods?