Try Vaseline. Rub a layer liberally all over your foot before you put your socks on in the morning, and leave it. My grandmother, God rest her soul, claimed to have seen this on a documentary on TV about the SAS. I was sceptical but gave it a go on my DoE Gold four day expedition across the Mournes way back in 1992. Turned out to be surprisingly effective - got a lot of laughs, but I was the only one who came home with no blisters.