Breaking in new hiking boots is easy and your boots can give you years of comfortable hiking, provided you take your time.
The first step in breaking in new boots is to select the right boots to begin with. It is essential to buy boots that really fit and feel comfortable and are appropriate for the type of hiking you plan to do. The harder the terrain, the tougher the boot you need. This may well mean you need to spend more money to get really good, appropriate and well-fitting hiking boots, but anyone who has suffered the blisters and pain caused by walking all day in ill-fitting hiking boots will attest to the fact that good boots are well worth the extra money.
Take time in the store to try BOTH boots on, and walk around for as long as you feel you need to. Try on several pairs. Make sure you are wearing proper hiking socks when you try the new boots, and ensure you can wriggle your toes in the boot. It is best to try boots on at the end of the day or after a hike or sports, when your feet are likely to be at their biggest.
Do not allow yourself to be rushed in trying on your boots, because a good pair of hiking boots will travel with you a long, long way, and there is nothing worse than being on a long hike in boots that are causing you agony. So take the time to find a well-made, comfortable pair of boots. Do not be tempted to go for cheap, ill-made boots, and do not buy boots that feel tight, hoping they will stretch and become comfortable later. They won't.
Once you have bought your boots you next need to soften up the leather. Hiking boots are tough, as they are supposed to be, and softening up the leather takes time and a little effort. One trick is to get a bar of soap and rub the insides of the boot uppers. (You can also use a special leather conditioner, but soap does just as well.) There are professional shoe stretchers and shoe stretching sprays, but soap is cheap and works just fine. Once the leather is softened, put on your hiking socks and boots and walk around the house/yard for an hour. Avoid over-tightening the laces. Even if your feet do not hurt after an hour, take the boots off, and leave them off until the next day.
Repeat the process, wearing the boots for an hour at a time. If you can, find a hill to walk up and down to make sure you do not slide forward or back in your boots. Gradually increase the length of time you wear the boots. When the stiffness is gone and you feel comfortable in them, go for a SHORT hike in the boots.
If there are no problems on your short hike, your boots are ready for a longer hike. Gradually increase the distance, but do not rush into going on a really long hike until you and your boots are old friends.
It is also important to look after your boots by using a good wax-based polish to keep the leather waterproof, conditioned, and soft. Look after your boots, and they will look after you.