What socks are best for ruck marching and what other tips are there to help with foot care during long distance rucking?

Any good thick sock to include the issue ones. Try to keep dry. Soak feet in brine or pickle juice a couple of times to harden skin. Use foot powder regularly, Have boot well broken in.

Toughen your feet up. I used to run around a lot with no shoes on and barefoot. Takes a bit to build those callouses up but once you have them you are good to go. Next is find a good foot power and apply it regularly. Now for socks. You are going to want to put out some money here on good quality socks. Some double up, some don't. Carry extra pairs of socks, you WILL need them.

Get two pairs of good quality boots and break them in good. I mean real good. This will be crucial and save you a fair amount of pain. Once you have broken them in, get a pair of good insoles, the memory foam kind and put them in there. This will help cushion your feet and help to wick away moisture and keep them in one spot rather than slipping and sliding around. This will also allow that you have a bit more for the laces to bite to.

Learn where your wear points are in your boots and if you need to add additional padding at those points to avoid getting blisters for as long as you can. Trust me, you WANT this. You will get blisters eventually, it happens but the longer you can avoid it the better. When you do get a blister do not ignore it, treat it but do it the right way. Do not rip it off, cut it or pop it. You want to basically treat it like a candle wick. Thread a needle and run it in one side, then out the other. Leave about an inch of material on either side and snip it off. The fluid will drain out and the skin that was puffed out will reseat slowly back into place. After a bit when it has healed just pull the thread out.

Learn how to balance your ruck load and pack ONLY the minimum necessary.

I love my fox rivers, and if its a particularly long ruck, make sure to change them whenever you can.