I tend to get on with everyone, and don't have blazing rows/major fallings out/long, simmering resentments/drama, etc. People tend to like me. I tend to make friends and keep them for life (so far). My relationships are stable and loving, and I stay good friends with exes.
Partly I think that's luck (I'm not saying I'm a saint) and partly I think it's:
I'm pretty good at working out people's motivations and why they are behaving in the way they are. Then I can either choose to make them feel happy and relaxed in my company or at least, brush it off. 99 times out of 100, how someone behaves is not about you.
Be generous with praise. Don't be insincere, but if you see something good in someone, point it out. I think the moment when my SIL began to truly like and trust me was when I witnessed her dealing with (what could have been a very embarrassing and public) tantrum with her 4 year old, and later finding a moment to say how masterfully she'd handled it, and how impressed with her parenting I was. Remind your friends of their triumphs.
I have a good model for how people should treat each other, and (usually) quickly work out people who should not be allowed to get any closer.
I have a pretty stable core of self esteem, and although I am always open to new friends, I don't need any one person to fill a void. I have put effort into doing things that make me personally feel happy and fulfilled, and it would take a fairly concerted effort from someone to knock that.
I am not overly invested in how people I'm not that close to see me, or in needing to be right/in charge/have power over people. I can therefore say "mmm, that's interesting" and not rise to bait.
I trust my instincts if I get an 'off' feeling about someone. Don't feel bad about not liking them. It's impossible to like everyone.
I was taught the secret of charm at an early age - it is not making people think you are amazing, it is making them feel amazing when they are with you. Start off treating everyone you meet as if they are worth 5 out of 5.
This isn't about people-pleasing -- although if you are a reformed people pleaser, like me, use those skills for good. You already have a finely tuned radar for other people's emotions, use it for understanding them.