Blisters form from friction between the foot and the boot. The friction causes the outer layer of skin to separate from the inner layer of skin; and when this does, fluid fills between the two and a blister emerges.
The best solution for blisters is prevention, and the best way to stop friction between the foot and boot is to have a properly fitted boot and two pairs of socks. The first pair of socks should be a thin liner of polypropylene, Orlon, or nylon and the second should be a thick wool outer sock. The idea behind two pairs of socks is that the rubbing will occur between the two socks and not on the skin.
It is also critical to break the boot in properly. Begin slow and work up gradually, paying close attention to the development of blisters. Once blisters do begin to emerge, treat the blister according to below. Then rub leather softener inside the boot where the discomfort is coming from. Remember the heavier the boot, the more breaking in it needs.
Blisters are exacerbated by wet feet. Damp feet equates to soft skin, which doesn't slide easily inside socks, causing friction. Thus, be sure to change your socks when yours are wet.
If a break in the skin begins to form, stop hiking as soon as feasible, remove your boots and let your feet air. Treat breakage of skin with tincture of benzoin and cover with moleskin or Second Skin. The idea behind the moleskin or Second Skin is that the moleskin will move inside the sock and eliminate the friction between the skin layers.
If a large blister has formed, remove the fluid inside before the skin breaks. To do this, first wash the area with soap and water. Then use a sterile needle to puncture the base of the blister and gently press the fluid out. Finally, protect the blister and area with a patch of moleskin or Second Skin that has a doughnut hole a little larger than the blister.
If the blister is already broken, wash the area, apply antiseptic ointment and cover the area with a sterile bandage.