On the backside of the edge stone, on the ground, trowel in some concrete, maybe halfway up the back of the stone, sloping down to the ground on a 45 degree angle. By using a trowel to to create the angle and giving the concrete a smooth finish, it will add additional strength to the backfill. Then after the concrete has dried, backfill with garden soil. By placing the concrete along the back edge, it reduces the movement of the edge to minor bumps or frost heave. If you are doing this in a high traffic area, or an area that may take some heavy abuse, creating a footing and seating the stones in the footing along with the sloping concrete backfill will create a very sturdy edge, similar to the way they are installing belgian block or granite curb stones along the streets of new luxury homes.
If you want a true natural look, it's tough to beat belgian block or granite curbing. Most stone yards will have access to different size blocks to suit your project. They may cost a little more than the manufactured concrete, but they are natural stone.