The secret for the longest life of a post in the ground is keeping water away from it. Water is the enemy!!!
I've never used Cypress, and I don't know anything about it first hand.
For every post that I put in the ground, I make sure that mound up the soil, or concrete around it. I want there to be a slope that carries water away from the post.
100% of every post that I've replaced that was rotted, the rot was right at ground level, and they all had a depression around the post to hold water. Sometimes it was from not enough concrete, or that lawn around the concrete built up over time leaving that area around the post as the low spot.
I keep reading about people putting gravel at the bottom of a post and I dont understand it. Gravel will create space for water to remain there longer. Having the post in solid concrete, or compacted earth means that there is less of a chance of water getting in there, and what does get in there is minimal. I would think that the gravel would create a cavity that would attract ground water to go into that area. I would never add gravel to the bottom of a hole for a post.
Depending on how much strength that I'm wanting out of the post, I will either dump the sack into the hole with some water already in there, and then dump more water in while adding more of the concrete mix right out of the sack. Then when the hole is full, I'll mix a little water with the concrete to create build it up higher then the surrounding soil. For a corner or gate post, or a structure like a gazebo or a porch, I mix the concrete first. Properly mixed concrete will be significantly stronger then just dumping it into the hole and adding water.