Different boots will require different amounts of break-in time. Lightweight models may feel perfect right out of the box, while heavier, all-leather models may require weeks to soften up and form to your feet.
NOTE: Most hiking boots stretch out slightly as they break in. But the break-in process will not turn a poor fit into a good one! Make sure the boots you buy feel snug yet comfortable before you take them home.
The basic break-in procedure
Begin by wearing your boots for short periods of time inside the house. Wear the kinds of socks you're likely to be wearing out on the trail. Lace your boots up tight, and make sure your tongues are lined up and the gusset material is folded flat. The creases you form as you break-in your boots will likely remain for the life of the boot.
Your new boots will be a little stiff at first, which is fine. But if you notice significant pinching, rubbing or pain right off the bat, you may want to take the boots back and try a different style.
If after several short indoor sessions your boots seem to fit comfortably, expand your horizons. Wear your new boots to the local store, around town or while working in the yard. Gradually increase the amount of time you spend in your boots and the distances you cover. Make sure your boots feel good at each stage before increasing your distance.
NOTE: Make sure your new boots fit comfortably before you can wear them outside!
Be vigilant throughout the break-in process for any pain or discomfort. As soon as you notice either, take the boots off. Remember -- small problems can become big ones very quickly. If everything feels good, try adding a little weight on your back as you hike, and/or hiking on more challenging trails.
If your boots feel good throughout the break-in process, but a single pinch or a hot spot remains, you may be able to correct the problem area by visiting a shoe-repair shop or your local REI store. Most have stretching devices that can help alleviate localized boot-fitting problems.
No such thing as a "quick fix"
There is no fast and easy method when it comes to breaking in new hiking boots. To do a good job, you have to put in the time.
Avoid "quick-fix" approaches like getting your boots soaking wet then walking long distances. They're too hard on your boots and they'll be murder on your feet. Also make sure you follow the manufacturer's care and water proofing instructions carefully.