In my experience as a communication coach, dealing with selfish people is not necessarily harder than dealing with other kind of people, it just requires understanding where they're coming from and adapting your strategy based on this knowledge.
Self-centered people can be dealt with effectively. Here are the ways to do so that I've discovered to generate the best results:
1. Understand that selfishness is not that bad. Being selfish starts from having a well developed interest in your own wellbeing. Sure, selfish people go over the board with this and often ignore the wellbeing of others, but in itself, looking after your own needs is actually a mature thing.
2. Understand what the selfish person wants. The only way you can get something from a self-centered person is if you give back something of value to them. In order to do this, you need to understand the specific needs that person has. So, take some time to get to know them, what they value and what they desire.
3. Show them what they win. Self-centered people will always ask themselves: "What's in it for me?" In order to collaborate with them, it's essential that you show them what they gain from it. It's not about your benefits when dealing with such people, it's only about theirs. Keep this in mind when trying to persuade them.
4. Keep your promises. If you promise a selfish person a certain benefit for doing something and you don't keep your word, you'll quickly lose their trust. And once you lose the trust of self-centered people, it's very hard to win it back. Don't just promise them benefits from doing something, make sure you keep your word about them.
The good news is that self-centered people often have a lot of value to offer.