Blisters

Training for my first HM, had to cut my run short today as blister forming below the ball of my foot. I had this problem before with my last trainers but went for analysis and got some new ones and everything was fine. That is until my last couple of runs where it has once again reared it's head. The trainers are very comfortable, using could cushioned running socks, two weeks to HM. Any suggestions ? Surgical spirit to toughen the skin ?? Vaseline ???

Cheers