When fusing sheets of glass together it is often desirable to reduce the air trapped between layers, thus reducing the air bubbles in the final piece.
There are several excellent and basic techniques for doing so:
Many kinds of glass have one side that is smoother than the other. By placing the glass sheets with smooth sides facing each other you eliminate spaces for air to be trapped when the sheets fuse together.
One of the simplest and most effective ways of reducing bubbles is to adjust your schedule so the top glass sheet is able to slump against the bottom sheet before the glass softens enough to stick and trap air. Start the squeeze about 100F above the annealing soak temperature and increase the temperature slowly (50-100F per hour) until you are at the slump temperature. Soak at that temperature for 30 minutes to an hour.
Place small pieces of frit every few inches around the edge of the bottom piece of glass. Rest the top piece of glass on top of these spacers. When fired, the center of the top sheet will sag first and the area of contact between the two sheets will spread from the middle — pushing the air out as it goes.
No, I'm not suggesting you curse at the glass. But by setting up heat dams around the perimeter of the sheets being fused you decrease the chance of the edges getting more heat than the center and "sealing" air between layers. Kiln furniture, strips of fiber board, and cut pieces of old kiln shelves all make excellent dams.
For the best results, consider combining any or all of the above. The use of spacers, for example, works best when combined with a schedule that includes a squeeze.