Unfortunately, this is not easy to answer, and highly dependent on the country. As discussed here for example in the context of young people for Japan. You will have to specify what type of people you want to meet and in which country you are to get a good answer with concrete points to follow on this.
Some general pointers that will drastically improve your success to improve this:
- meet people with similar interests, so you have something to talk about.
- be prepared to invite people for a drink at least.
- plan ahead. Get in contact as much as possible before your trip.
- Social Venues: Bars, concerts, festivals. This of course depends on your ability to socialize. If you are not good at chatting up people in your own country in such locations, you might have the same issues abroad.
- Interest groups: If you have special interests such as photography, comics, specific sports and a reasonable experience with such topics, you can try to find clubs, associations beforehand online and ask if you can meet some likeminded people at your destination and invite them for a beer. The further away you come from, the more likely it is they will invite you with open arms.
- Social networks, industry associations etc: There are places like meetup and interest groups that are more formalized that allow you to meet people. Again, you would have to plan ahead, but a high chance that you meet people. Other organizations such as Rotary clubs are also always a good thing to join if you travel a lot and want to meet people.
If you are only out to meet someone for a single ad-hoc lunch or dinner, you are in a completely different field of course. My strategy is to go to a restaurant where you can sit at a counter, so you can at least talk with the chef behind it, and better even to the people sitting next to you. Also restaurants that have a large community-table are good for this. Last but not least, checking if there is a soccer/whatever-match and going to a sports-bar with big screens is a good way to meet other people and eat in a more social environment helps a lot.
If you travel often for business to the same town, it helps a lot to always go to the same places. It might be more boring food-wise, but you will realize that the guests also come back to the same place and get to be known and know people. That also would require that you avoid hotel bars at all cost since there you will only meet other random people that are unlikely to be back there when you return.