Design your work to minimise the possibilities of trapped air.
One way to do this is to use strips. Lay thin strips of glass on edge and fuse these together, instead of layers stacked on each other.
Another is to design work with many smaller pieces, rather than large ones. These create more pathways for air to escape.
Some advocate cutting the bottom layer in several strips to allow the air exit spaces from between the glass layers.
Note that all these methods leave marks of where the edges of the cut glass was, so they need to be planned to fit with the design.
In general terms, you need to think about how the air will move out of the piece. Are there places where there is no escape for the air? Allow a channel for the air to move from the centre to the outside.
Glass arrangement considerations