I have been using a ruined 1" flex bit that's six feet long. I sleeve the bit's shaft with a piece of 1/2" PVC and drill away. The bit cuts a hole and the PVC just stays there, but it keeps the path straight. If a rock is encountered that might make the bit move, we know right away since the PVC won't follow the path if if's not straight. We unchuck the bit when done and pull it through from the other side of the pipe when we are done. Of course, this only works for sidewalks, etc. but it works well and it's clean.
