Sock liners are much thinner than walking socks and are made from a smooth synthetic fibre designed to wick perspiration away from the foot and minimise rubbing that could cause blisters.
We used '1000 Mile' sock liners. These are quite expensive costing in the region of £8 a pair. However they are actually two socks in one, in other words the sock liner has a liner as well.
We found them to be both comfortable and effective.