STEP 1: The first and most important step in successfully sabering a bottle of Champagne is to get the bottle (and more importantly the neck of the bottle) extremely cold. I do using a Champagne bucket filled with ice-water. After you are sure the sparkling wine is sufficiently chilled, invert the bottle neck down and chill for an additional 15-20 minutes. By that time, the neck will be the coldest part of the bottle and that will insure success.
STEP 2: Remove the foil and the cage from the bottle. I remover all the metal foil and the cage from the bottle leaving the bottle bare, with the exception of the label. This will make it easier to find the seams that run down the side.
STEP 3: A Champagne bottle is made from 3 seperate pieces of glass that are then fused together. There are two side pieces and one top or cap piece. If you look closely you can see the side seams that run along the glass where the two pieces were fused together. If you follow the side seam up to the cap piece at the top of neck, you will notice the lines of fused glass form a T just below the lip of the cap. Where these three pieces of glass come together at the T, the bottle is very weak. Especially when it is cold. (Hint: don't rub the seams of the glass with your finger. You will warm up the glass making it harder to get a clean cut and also wiping away the condensation will actually make it harder to see the line not easier.)
STEP 4: Next you want to get a good grip on the bottom of the bottle and get ready to sabre. I sometimes use a towl to help hold the bottle if it is slippery. Hold the bottle at arms length away from you and tilt it so that is slightly angled upward. Make sure that the seam is directly on top. Place the sabre with the blade lying flat on the seam. (you can use the back of a heavy chefs knife if you don't have a saber) You want to slide the blade along the seam so that it strikes under the lip of the cap piece directly where the three seams of glass come together.
STEP 5: Make sure the area in front of you is clear and let er rip!! You will be surprised how easily the neck breaks and the cork with glass cap piece still attached will fly into the air. This is not a test of strength. A well placed knock to the weak part of a chilled bottle will do the trick. The glass should break smoothly and because the wine is under so much pressure glass will never fall back into the bottle. You shouldn't lose much if any Champagne from the bottle and you can savor your sabre.
STEP 6: Enjoy your bubbly and find your souvenier! The top of your first sabered bottle is a nice keepsake. Be careful when pouring, the bottle is sharp where the glass has been broken but the wine is completely safe to drink.