Avoid lining them up in a straight row. For me the appealing forest will maintain somewhat of a triangular shape in the tree line. That is, the thickest, tallest "dominant" tree toward the center of the group, the others dropping in height toward the left and right. Nothing like a perfect triangle, mind you, but generally like one.
I'd suggest looking at examples for inspiration, like these:
http://www.bonsai4me.com/Gallery/Ga...igCoussins.html