Other folks advocate traveling to the area or region you are going to be writing about to get a taste of the local flavor and to interact with the locals. I'm not really the cheerleader for this movement either. Charleston, South Carolina is perhaps my favorite city and I would hate to have a weekend visitor attempt to a paint an accurate image of the city scape. I just don't think a weekend in a locale gives you much beyond what the tourist brochures provide.
I have found a method that works for me. First, I write about geographical areas I know. I want to make sure that I'm 100% accurate when I am talking about a place. I want to make even the locals find something new to them. Fortunately I've traveled a good deal otherwise I would be limited as to where I had available to me as settings. I believe that if you write about an area you know then it will show.
Now when I need to know facts, the nuances of a profession or things such as techniques, procedural matters or otherwise, I just ask. I have found that most people with a particular skill set or specific knowledge are more than happy to have an eager audience who is willing to listen. In the off case where a person will not talk to you, I have two techniques that generally will get them to speak to you. One, do a little research in advance and learn who is recognized as another "expert" in the area. If they don't want to speak to you simply say something like, "Thank you so much for your time. I had one final question, as a back up I am going to call Mr. or Ms. So and So. Is it okay that I tell them I have spoken to you?" Or, secondly, try this. "Thank you so much for your time. You're the first person I called and wanted to see if you could suggest someone I could speak to who would be as knowledgable as you on this subject." Generally one of those two will either get the person talking or they will give you a promising lead. Just keep in mind, it is a rarity that people won't talk about themselves and what they do.
One additional point, if you know someone who has a personal relationship the individual you need to speak to, by all means take advantage.
Remember, don't be worried about rejection. If you're rejected, try again. The simplest thing you can do is just ask. However, a word of caution, don't get bogged down in the research. After all, the goal is to write and if you are researching, you're not writing.