Here's a quick Instructable on how to break in Army-style leather boots. All of the boots shown have rough leather on the outside but any of these processes can be used for smooth leather hiking or work boots.
Step 1: Step 1- Water
One of the easiest and fastest methods is to use water. The trick is to use water along with regular walking to help loosen up the leather. This may seem counter intuitive because water is known to shrink leather but often times the leather used in boots is pre-shrunk and won't get any smaller.
In the morning as you are getting ready to go about your day, head to the nearest sink, bathtub, shower, etc and fill up your boots with water. Fill it up as you would a cup and then dump it out. The point is to get the boot soaking wet. If your boot has drainage holes, it may not fill up but just make sure its completely drenched.
Next, put on two pairs of socks. Having two pairs helps prevent blisters and acts like a moisture reservoir to keep the boots wet all day.
Finally, lace up and go about your normal business. Just wear them around all day as you would normal shoes. The water may feel a bit strange at first but it's not that bad. You may want to re-tie about an hour later or so because the leather will start getting loose quickly and it helps break them in faster if they are tied tightly.
When you get home, open up the laces, pull out the insoles, and throw them in front of a fan to dry. Gore-Tex boots tend to take a lot longer to dry than regular boots but they will still probably dry overnight in front of a fan.
Depending on your type of boot, you may want to wear them wet a couple of times. You should be able to tell a difference in flexibility and stiffness from the new boots before moving to the next step.