It is difficult for the seams not to show. You can fire the pieces “face down”, which may help (and then flip and fire a second time to polish). You can lay out the piece with three or more layers and dam. The best method is to overlap — where the seams meet, cap with another piece of the same color. So… you are basically making two layers out of the color rather than one.
The best solution is not to have seams (or have intentional seams that are part of the design).
Jester’s Baubles Fused Glass Designs