Using a candle and some patience, carefully heat the bottle at the scoring mark rotating the bottle as you heat it. Hold the bottle with both hands, one on each end. I will typically heat the bottle for about a minute and then drip cold water on the area. Repeat the process until the bottle suddenly breaks apart. You will actually hear slight cracking of the glass as you do this.
The first time I did it I was very tentative but now it usually takes me only about 5 mins to cut a bottle.
Be carefull, the cut edges are EXTREMELY SHARP!
*Note: I've also had success by heating the bottle in the oven for about 5 minutes at 225 F and pouring water over the score line. You'll need to ruin a few bottles to perfect your method.