1. Understand where they're coming from.
By understanding what motivates their negative behavior, you should be able to make dealing with them a lot easier. Are they simply being cruel and inconsiderate, or does their selfishness stem from fear or anxiety? Gathering insight about the root cause of their selfishness will help you respond in a way which doesn't aggravate the situation.
2. Don't take it personally.
Even if selfish behavior makes you tremble with rage, you should always try your best not to take it personally. Always keep in mind that a selfish person is somebody with a persistent underlying problem. If they question a decision that you believe is right, then it's probably because they're thinking of how it will impact them, and aren't taking your thoughts and feelings into consideration.
3. Avoid being defensive.
Try not to get defensive when a selfish person criticizes you. Take everything they say with a pinch of salt, and treat them as yet another uncredible source. Getting mad will only make things worse.
4. Realize that selfishness is healthy sometimes.
Once in a while, it's good to be a bit selfish and do something for yourself. Constantly working for and supporting others can be draining, and can also set you up to be abused. The biggest issue arises when selfishness is not just an necessary escape, but becomes an entire way of life.
5. Set boundaries.
In order to protect yourself from selfish people, you need to set distinctly identifiable boundaries. Speak up for yourself as soon as you feel uncomfortable, and keep in mind that people are more likely to act selfishly if you let them keep getting away with it.
6. Don't feed their selfishness.
If you have to deal with a selfish person on a daily basis, the last thing you want is to be actively making their flaws worse. Never praise or reward them for anything they obtained selfishly, and never give in to their selfish demands.
7. Retain your own personality.
You should never change for anybody but yourself, and this is especially true when it comes to dealing with self-centered people. If you do, what's to keep them from walking away from you, leaving you permanently changed for the worse? Concentrate on your own value, and spend time with people whose positivity will rub off on you.
8. Be patient.
Always try to remain calm, and avoid overreacting. A lot of selfish individuals have undergone past experiences which have caused them to develop dysfunctional self-preservation techniques. By viewing them with compassion instead of contempt, you can continue to hold them accountable for their actions, without aggravating the situation.
9. Decide if they're worth keeping in your life.
If the person in question is a family member then this decision will be incredibly tough to take, yet in extreme cases it may be for the best. If this person is reducing your quality of life, and you've tried everything else in this guide, then cutting all contact might be the only way forward.