It really helps if you have a way to show that you are both respectful and serious.
It is hard when you are just starting, but once you have a few good results, you can put them into a "proof" book and show them to new people, who are then more likely to consider seriously your request.
It probably helps to have a working title. A business card too.
If you are shooting for a news publication, you can rely on a press pass.