Learn the local language. If you will be traveling internationally, few things go so far towards meeting local people while traveling as learning their language. Of course, the more fluent you are, the more people you will be able to talk to, and the greater range of subjects you will be able to discuss. However, many people around the world do speak English, but will still feel more accommodating if you have made the effort to learn at least a few phrases of their language.
Participate in local programs. Compared to regular tourism or backpacking, many organized programs provide a greater opportunity for meeting local people while traveling. Some examples of these programs include joining volunteer organizations, doing study abroad courses, or arranging for a home stay with a local family.
Use your connections. If you know someone who knows someone where you will be going, see if they would be interested in meeting up. Don't put pressure on them for accommodation or other services, but suggest something like taking them out for coffee so they can give you some tips about the area. You may hit it off, or they may be able to introduce you to some other people.
Sign up for a group tour. These vary a great deal in scope. You could choose an entire package trip that would have you traveling overland or by sea with the same group of people the entire time, or you could do something as simple as an afternoon walking tour. Many of these trips vary greatly in terms of target audience, but chances are if you pick one that interests you, you will meet other people who share your interests.
Hang out in traveler haunts. These places generally have a deservedly mixed reputation, but especially if you're on an extended trip they can be a comfortable break from foreign experiences. Plus, most travelers love meeting other travelers, and there are few better ways to do this. Try staying in hostels or visiting clubs, restaurants or cafes that cater to expatriates.
