Once you have bubbles trapped in your glass there are no real easy fixes. You can break the bubbles open and pour frit into them. You need to pile the frit a little higher then the glass. Sometimes you will still end up with little bubbles in your piece.

 

 One way to avoid trapping bubbles in the glass (especially if you have glass pieces suspended between two or more layers) is to prop up the outer edges with glass frit. This allows the glass to sag in and all around the middle pieces. Then the glass seals its self from the center to the edge, allowing the air to escape. The cons to this method are that you often have to grind the outer edge, due to the excess glass frit you added to prop up the lip.