I used somthing simular Daiv, I cemented them in the ground directly below the post, cut them a little below the soil level and drilled a 1/4 in hole in the top of them and bolted my posts to them. In my case, I recycled a swap meet canopy frame and screwed plastic patio cover roofing to it. The frame is made of 2 in galvanized tubes that I also drilled to accept the through bolt in the bottom. Hey, its not pretty, but it works.
You could do the same thing with the wood post's but would probably not want the wood posts in contact with the ground, maybe set them up on a brick slightly above the soil line?
I guarantee the corner post's wont budge! Dont know about the rest of the structure though.