http://www.signs101.com/forums/showthread.php?64145-need-your-input
I like making these T-Frames for easy-to-install MDO signs. No digging necessary, and completely re-usable.
Pics/Diagram/Description in that thread, but the legs are basically a "T" of 4x4's supported with 45degree 2x4's. drill holes in the legs and pound 2-3ft of rebar through it and into the ground.
noted: that picture in the thread was of a frame I had designed on site to work with the slope of the hill it was installed on. I'll usually attach all my legs on one plane, like the diagram in that same thread.