These are things you can use to prevent blisters or help mitigate the pain of an already formed blister.
The best way to deal with blisters is to catch them before they form. If you know that a particular pair of boots gives you blisters then you should take measures before you start out. For example when I wear my leather boots I always get a blister on the back of my right ankle. Now days I put moleskin on that area before I ever put my boot on. Once on the trail you need to see if you feel any "hot spots" or rubbing in your boots.
Moleskin:
For preventative steps the best material to use is Moleskin. This is a
thinly padded, adhesive, tough pad. Moleskin is probably about 2mm
thick, or about twice as thick as a standard bandaid. One side is
adhesive with a peel-off backing to expose the sticky surface. The
non-sticky side is a soft cloth surface.
To apply moleskin first cut a piece of appropriate size. You'll want it large enough to completely cover any blister prone area. However you don't want it too big because as the two dimensional moleskin wraps over your three dimensional foot you will introduce folds into the moleskin. These can cause problems on their own so you want to keep them to a minimum.
You can apply moleskin directly to your skin, but I always apply Benzoin first. This is kind of an antiseptic glue. You find it in drug stores near Iodine. The thing about Benzoin is that it's kind of difficult to work with. It's very un-viscous (like alcohol), but it's also very sticky. It also permanently stains any material it gets on. One last thing about Benzoin is that since it's like glue it eventually permanently seals closed the contain you keep it in.
Despite that I've been using Benzoin for blisters for probably twenty years. I find moleskin much more likely to stick longer when I've first applied Benzoin. When I put on moleskin I usually put a finger over the tiny Nalgene plastic jar I keep my Benzoin in. I tip the jar to get a fingertip full of Benzoin. I spread that over the area that gets the blister, I let it dry for about a minute, and then I apply the moleskin over the Benzoin. I first cut off the corner tips of the moleskin to make it more circular so there are no tips to get easily lifted up.
Molefoam:
If preventative measures have failed, and you've managed to get a
blister then your aim will be more to mitigate pain (as well as trying
to prevent the blister from getting worse). In this case the best thing
to use is Molefoam instead of Moleskin. Molefoam is about a half
centimeter thick of actual foam with an adhesive backing (just like
moleskin's adhesive backing). The idea is to use the molefoams
thickness to build a wall around the blister. Your sock and boot then
press against this wall instead of the blister.
Scholl's also makes molefoam corn rings. These are rings of molefoam which you center over the blister. With either corn rings or regular molefoam the idea is to build up around the blister, but to leave the blister itself uncovered. I sometimes put a bit of gauze over the blister itself inside the ring of molefoam. Again, I use Benzoin to prep the skin for better adhesion. Benzoin on the blister will sting, but it's an antiseptic like iodine so there's no problem with getting Benzoin on an exposed blister. If you do, however, then you should definitely put a little gauze on it so the Benzoin doesn't glue your sock to your blister.
2nd Skin:
Finally, if you have a blister that is so painful that isolating it with
molefoam is not working, or if for some reason you can't isolate the
blister then you'll want to use 2nd Skin. This is a strip of some sort
of gel about 2mm thick. I have seen two different kinds of 2nd Skin.
With one kind there are backing covers on both sides of the second skin.
You remove one side, and apply it directly over the blister (broken open
or not). Then you remover the outer backing. Finally the gel is
covered with a very very thin kind of moleskin (it's really more like
just the adhesive strip of a band-aid, but it comes with the 2nd Skin
kit). 2nd Skin application has enough steps that it's worth bringing
the instructions with you.
I have used Benzoin before when using 2nd Skin, and I've put 2nd Skin on without Benzoin. Both seemed to work equally well, but I usually put it on out of habit. More blisters will be needed for further study to see if Benzoin is worth it in this case.
Finally Dr. Scholl's makes a 2nd Skin-like gel pad. I used these on my SB02 trip after I carelessly got two large blisters. The Dr. Scholl's pads were firmer than 2nd Skin brand, and the blisters just got worse underneath the gel pads. Thus so far Dr. Scholl's gel pads do not get a nod from me, but I'm going to try then again some time so make sure it wasn't user error.