How should pressure treated wood post foundations be protected?
I live in the Pacific Northwest of the US where it is also fairly damp. In 2000 my neighbor and I built a 140 ft long stretch of fence between our properties. For materials we using 6"x6" 8ft pressure treated Douglas fir posts every 10 ft with a pair of 2"x6" 10ft p.t. cross members between each. The posts were rated for ground contact against insects and rot and came with a lifetime warranty.

After doing a fair amount of research, we ended up digging 3 ft deep holes and put about 4 inches of crushed gravel and a few hand-size rocks in the bottom of each. This is to raise the post above any standing water that might accumulate there. Since it's also very windy here certain times of the year and it tends to blow perpendicular to direction of the fence, we filled the holes with concrete. 

Probably a bit of overkill, but it was so much work—especially digging the post holes in the rocky ground—we just wanted make sure it was done right. That's going on 17 years ago now and it's still standing and remains very sturdy.