How do I cut a zip tie that is really tight on a few cables?

[Scheer]

I always use flush cutters: http://www.amazon.com/Klein-D275-5-5-Inch-Lightweight-Cutter/dp/B000GTMZHG/ref=sr_1_2?ie=UTF8&qid=1426258865&sr=8-2&keywords=flush+cuts

 

These are also what you want to use when cutting the tails off the ties when you install them, the diagonal pliers mentioned above will still leave a bit of the tail sticking out since they are sharpened from both directions, and those tail stubs can be sharp and cut you.