The trick on any wood post in the ground is to keep them dry. Make sure that the soil slopes away from them and there is no standing water. I repair fences all the time, and in every case, water sits at the fence post and rots it away there. Even if the water is gone in a day or a few hours, it's that moisture that causes the rot. Add eves and gutters if you can. Anything to get the water away from those posts is your most important goal in logevity of the poles.
Good luck,
Eddie