I've heard it suggested to cut the two pieces that you want to join...
1.at the same time, by overlapping one on top of the other just a little ways in, like maybe an inch?
2.with a rotary cutter
3.in a long curvy line.
Then butt those two curvy lines up against each other instead of just butting two straight lines up against each other. And zigzag just as was suggested above. I've heard the curvy line helps to eliminate bunching at the point where it's zigzagged together.