In the FS district I work for and in the other districts in the Wenatchee/Okanogan National Forest we use what I thought was a standardized method.
We have plastic diamonds. They are nailed to trees with 16 penny galvy nails in such a manner that as the tree grows the diamond is pushed along the nails. In other words the nails are not driven home. 4X and Moto trails share the same color, grey, Cross country ski/snowshoe trails are blue and snowmobile trails are marked orange. The idea is to mount the diamonds so that you can start to see the next one as you go past one. They are sized at about 4" per side which makes about a 6" diamond. They are avaiable commercially. Also can be gotten with a reflective surface. And there is a larger size which can also have an arrow, left, right of straight.
Tomorrow I will get the order info. I think off the top of my head they came from "Rockart" http://www.rockartsigns.com/
I'll also get a couple pic's. We also have a trail designation explantation at numerous Kiosk and trailheads thoughout the forest. If the FS is asking have them contact me I'm with the Naches Ranger District.
Hope this helps