Heard a contractor say once that clients often have no choice because of property laws or animal control. They need that line marked even if the ground is awful. Read about using screw-in metal anchors instead of wooden posts in really wet areas. You sink them deep and they grip better in soft soil. Another idea is to pack gravel around the base before adding dirt for better drainage. It's extra work but might stop the leaning over time.
Feel your pain on this one. Clients sometimes fixate on marking boundaries no matter what. I've seen posts wobble like jelly in that muck. Those screw-in anchors kim mentioned can help, but it's still a headache. Honestly, half the job is managing expectations when the ground fights back.
Gravel can actually make drainage worse in pure clay, it just creates a hidden pool. That soil type really needs a different fix, like a wider concrete base to spread the weight. It's one of those things you learn the hard way after a few failed posts.