This video is pretty informative for lining out the proper way to install timber poles.
I especially thought the '10% +2 feet' rule was handy for determining just how much of the length should be planted into the soil:
My experience has been quite a bit messier, with engineers calculating the exact depth based on a soil survey. However, this rule of thumb is really close to the calculated result in my recollection!
Also, there are some good tips about how to read the markings on a pole, understanding the 'rating', or acceptable size vs. mounted equipment weight limit, and an overview of the installation process.