As the owner of a specialty outdoor store, I get asked the question "how do I break in new hiking boots" quite a lot. Just as the materials, techniques, and styles of hiking footwear have changed so has the answer to this question. Today, many types of hiking footwear are wearable "out of the box." I'll discuss the issue in more detail below, but the rule of thumb is the heavier duty the boot, the more breaking in you will need to do.
It used to be that hiking boots were hiking boots. They were big, heavy, had thick soles, and were made from large, thick pieces of leather. These boots had to be broken in before used on an extended hike or trip, or else your feet would pay the consequences. These days, however, such boots are few and far between.
Today's outdoor store is stocked with several categories of hiking footwear, including: trail runner, approach, light hiker, midweight hiker, backpacker, heavy backpacker, and mountaineering. Typically, the footwear from trail runner to midweight hikers are wearable out of the box. In other words, a quality shoe or boot in one of these categories should not require "breaking in" before you hit the trail.
This doesn't mean, however, that you shouldn't get used to the shoe or boot ahead of time. You'll want to make sure the footwear is truly comfortable before you take it on the trail, with the same type of socks and inserts, if any, you would use on the trail. The short amount of time you spend in the outdoor store is simply not enough to determine if the shoe is a truly good fit. Walk up and down the stairs a few times, wear them while puttering around the house and watching t.v. You're not really breaking them in, but testing them out. Still, the wear will get your feet used to your shoe or boot and vice versa.
On the other hand, backpacking, heavy backpacking, and mountaineering boots will need a period of breaking in before significant trail use. The heavier the material used, and the fewer the seams present, the more breaking in you will need to do.
To start, wear your boots around the house for short periods. Make sure you are wearing the socks and liners you would wear on the trail. Also make sure the tongue is correctly placed and the boot is laced the same way you will lace it when hiking. The molding you force your boot to do during the breaking in period will likely last the life of the boot.
Your boots should feel stiff, but not painful. If you feel heat in your heal or ball of your foot, or the sides of your foot are pressed firmly against the sides of your boot, you might want to investigate a larger size. Stiff boots will mold to your foot, ill-fitting boots will mold your foot; the former is fine, the latter ruins trips.
After you wear the boots around the house a few times and you've determined that they are the correct size, take the boots for a walk outside. Gradually increase the time and distance you wear them. Once the boots begin to feel comfortable, start adding weight, such as taking a day pack with you.
Breaking in takes time to do properly. Not only are you breaking in your boots, but you're acclimating your feet. Done right, you should have no problems once you hit the trail.
If you absolutely don't have time to break in a pair of boots, but need a new pair for an upcoming trip, try to get a pair that is wearable out of the box. In other words, instead of a backpacking boot, try a midweight hiker. It might not give you as much support, but it may make up for the difference by keeping your feet healthy. If you insist on backpacking boots, you have three options: 1) go to a shoe shop with a mechanical stretcher, 2) wet your boots and walk in them as long as you can stand it, letting them dry on your feet (I know someone who has done this with marginal success), or 3) go without breaking in and practice your blister prevention and first aid.
Most importantly, happy trails!