When I first got the gardening itch, I ended up utilizing some fresh sawn 2" pine to create a number of beds in various sizes. These have held up well over the last three-four years with the failure points generally being around where the fasteners were installed. All of the beds are still standing in decent condition.
Now adays, I just create raised beds by piling materials up without anything delineating the sides. I woodchip around these spaces to keep the grass out and I have had great luck so far growing in them.