Aside for a short stint with an agency in 2006, I do everything for myself. Photojournalism is a tough area, with opportunities shrinking, and (if you can afford it) it is best to do the work which genuinely interests you than pick up marginal assignments. Remember, old press is dying, new one is not anywhere in sight :)
One thing to distinguish is long-term, documentary work and current events. The latter is basically reportage, the bread and butter of news agencies and newspapers. On the former though, you can choose your ground and take your time.
I start a project with pulling all information I can from the web: saves time on the ground. Names, history, maps, prior works in this area. If necessary finding the initial contacts. Over there, it's approaching people, asking questions (not necessarily identifying yourself as a photographer), you can get a lot of context just from casual chit-chat. Good thing is that most people love being heard, so give them all attention you can. Start a neutral conversation, listen, nod, let them pour out: very few people are generous listeners, so your attention will be appreciated and you can ask the questions you want afterward.
In case of hot events, various police and security troops are a major challenge and great test of your social skills. There it's best if you can start small, attending fenced but calm events, just to get the gist of unspoken rules, protocols and routines. Obviously you can't generalize this worldwide, but. Be attentive, observe police, observe protesters or other people interacting with law enforcement, see how much can work with them. Read faces, in case of militias face is a good indicator how much problem you are going to have with a given person. If he looks like a scumbag he probably is. There are weak points in most of cordons and often ways to sneak past fences. It is best if you stay calm, neutral, and project confidence; especially confident, you can pull nearly Jedi-like tricks with it. Do not insist on eye contact but do not avoid it either. Don't smile, it shows your weakness and reduces your chances to successfully communicating with people in power dramatically.
It may sound hard but the skills build up quickly once you start doing.
