After purchasing full leather boots (similar to these boots) I had many blisters for the first few hikes until I broke in my boots.
What are the fastest ways to age the boot so that when my foot won't blister every time my feet slip into them?
One of the things I've heard that wildland firefighters like to do (they often wear large, all leather boots, like these: Danner Flashpoint II) is put on the boots, stand in the bathtub with water and let the water soak through the boots with your feet on, and then wear them around the house for a few hours. It seems to work - as it softens the boots and helps to break them in faster.
Blisters usually form when your socks get sweaty and things start to rub around.
When I first bought my pair of boots, the man in the store told me to wear them around the house for an hour every night for a week or two before my trip. This gives you a chance to break in the leather slowly over time, while keeping your feet blister-free.
Obviously, this won't work for last-minute boot purchases, but I'd still recommend you try to wear them around for a few hours in a casual, no sweat environment before you head out.
I have a feeling this question is going to come up a lot on this site... as I mentioned here it is an old and persistent myth that full leather boots will necessarily have a blister-inducing break-in period.
Well-built boots that fit properly can be broken in fairly easily and blister-free. Hot-spots and rubbing are usually an indication of a poor fit (toe-box too narrow or wide, boot too long / short, too much volume around the bridge, too much volume around the ankle etc...)
If you need a pair of heavy full-leather boots, your best bet is to get properly fitted by someone who knows boots (not just some clueless yahoo at the boot store), and try as many different brands as you can. Each brand uses a different last - or mold - of a "normal" foot on which their boots are built. If their version of "normal" is too far off from your own normal foot, it won't matter what size they are, they won't ever fit right.
When I bought my leather boots they told me that modern walking boots should not need breaking in if they fit properly. I followed their advice and bought well-fitting boots and did not break them in. The boots have never given me blisters despite spending many days walking in the mountains in them.
This may be helped by my choice to wear two layers of socks one thin and one thick over the top.
The single best thing you can do to avoid blisters is making the right choice of boots in the first place. If they are good quality, fit really well and suitable for your chosen activity I wouldn't expect many problems.
Go to a specialist outdoor shop and get advice from an expert. Most reputable outdoors shops will allow you to buy them, try out indoors at home and exchange them if they aren't suitable (at least in the U.K. and assuming they are still in a new condition).
If your boots are still causing blisters after a few hikes, then perhaps you've got the wrong boots. Other problems such as a large, badly packed or unbalanced backpack load can affect your gait and cause problems too.
Having said that, I've bought the wrong boots in the past and for me, soaking them in a river, then walking them dry worked well.
Using some sort of leather conditioner, work the boots with your hands. Flex the toe bend and ankle areas. Using your thumbs apply very firm pressure to the toebox in circles. Flex it back and forth as much as possible.
Leather is made supple by use and conditioning. The conditioner will vary based on your boot. Some examples are Lexol, Neatsfoot Oil, and Mink oil. In the really old days this was done by tallow (or other fat) and chewing, followed by tallow and rubbing. Now most leather is machine conditioned at the hide level, and somewhat further at later steps, but the manufacturing and time on a shelf makes it stiffen up more.
Time to condition is also affected by how good the fit is to start with, at least to the degree that we're talking about conditioning for comfort. Whenever I buy boots I immediately go out and hike five miles (preferably someplace that won't beat the shoes up). I return the boots if I'm not feeling good with them at the end of that five miles. Walking around in the store is just not enough to verify fit.