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I posted a short guide to champagne-sabreing
technique to MetaFilter.
You just never know when a skill like that will come in handy.

Dan’s champagne-slicing quick reference

1. Hold the bottle (in your non-dominant hand) pointing away from
   you and tilted upward about 30°.

2. Hold the knife in your
   dominant hand, level with the horizon, blade away from you. Place the
   flat of the knife on the surface of the bottle (on or around the
   label).

3. In one smooth, firm, confident motion (believe
   me, this helps), sweep the knife down the length of the bottle,
   maintaining bottle contact the entire way. Imagine that you will slice
   cleanly through the neck of the bottle, because this is what will
   (hopefully) occur.

The pressure of the champagne is
focused on a weak point in the glass of the neck of the bottle, and it
(the neck) will split along invisible lines of cleavage. You’ll be
left with some champagne on the ground, the bottle neck somewhere
across the room, and a bottle of champagne ending in an elliptical
conic section, ready to be poured out to your admiring friends.