People who make me feel good is mostly about the things they don't do, more than anything.
I CBA with people who flake out on meetups for pathetic reasons like feeling tired, hungover, couldn't get childcare because you didn't try until last minute, had a better offer, decided to give your mum a lift to your auntie's house because she didn't feel like getting the bus etc etc. Like honestly, just organise your life so you can attend the things you've committed yourself to. Respect the other person's time and the fact that they may have had to turn things down or shift arrangements around to accommodate this meetup too. If something goes unexpectedly tits up with your life/week cancel in good time, not the day before (unless you genuinely didn't know there'd be a problem until then) or worse, 5min before you're due to be there. Also, grow a spine so you don't say "yes, absolutely, I'd love to!" whilst secretly thinking "help, how do I get out of this?!" It's easy, just say "no thanks" at the outset. Any hurt feelings are going to be much less than if you flake out last minute.
Don't ask to borrow money and give an unrealistic timescale for repaying. If you can't give it back until 6 months time, say so, don't say you'll repay it next week knowing you can't then avoid the person's calls for 6 months. Consider your debts to friends a priority after food and other essentials, not an optional extra that you might repay after you've paid the deposit on your summer holidays. And don't be scrounging every month like your mates are a second overdraft facility.
Have opinions. Nothing worse than someone who always says they don't mind, to absolutely everything. Except for those who think it's their mission in life to convert you into agreeing with their opinion. It's ok to disagree, you know, and not be clones of each other.
Friendships should be fun. We all have problems and we all need a moan now and then. Moaning endlessly about some self inflicted situation that you could change but won't take steps to do so, is another matter. Don't be that person who only contacts your mates when you want help/advice/to vent, arrange fun stuff too.
You needn't live in each other's pockets, but check in. If you haven't heard from them in 6 months it's best to ensure they're still alive and not going through some horrendously bad stuff but unable to reach out. Liking someone's pics on Facebook doesn't count as being friends, nor does a status update post of what you've been upto. They're your mates not your entourage, they don't spend their lives checking social media to see what you're upto, contact them with your news and ask for theirs.
Keep people's secrets. Keep their personal stuff private. Nobody wants to be the subject of gossip. If you want people to socialize in a group with your/their partner, maybe don't tell them every little nitty gritty bit of every single marriage problem so they hate your man. It's also going to be awkward next time I meet your cousin now you've told me her last baby was taken into care. Oh me? I'm totally fine, nothing to see here! (Not letting you shoot your mouth off about me to all and sundry). Without any trust it's difficult to be true friends.
If somebody has a bereavement/breakup/mental health crisis, don't leave them alone and uncontacted until months have passed and you think they're over it. It's fucking cruel. One day, you'll be the devestated person going through a tough time and you'll want your mates to check in on you and cheer you up a bit by distracting you with everyday normality.
It's not rocket science. Just don't be a fucking twat, basically.