Be on time
I try really hard to be on time and consequentially, spend a lot of time waiting for people who are late. The epidemic of young people being late is at atrocious proportions. This is all I need to say about being late:
There is no such thing as fashionably late before 8pm or when someone is cooking.
When people are always having to wait for you you're sending one of two messages. You don't value other peoples time and/or you can't get your shit together. Neither is a good look.
Seriously, show up on time. To everything.
Respect boundaries in conversation
I am an over-sharer, and I come from a long line of over-sharers so I have to be particularly careful about this. A good rule of thumb is to avoid talking about health issues, money, and intense emotional experiences, yours or others, in casual conversation. NEVER talk about your sex life in mixed company. I am astounded by how many people put that stuff on blast. It's not a sex negative thing, its that most people are uncomfortable hearing that information from people other than partners and close friends. And it's extremely disrespectful to your partner/partners to make something they thought was private public info. I am particularly distressed when people in monogamous relationships do this because we all know who you're talking about and it's super weird. If you think something will be okay but aren't sure you can say something like "is it okay if I ask how much that cost?" or "do you mind if I tell a story about my weird gynecologist who looks like Mike Dukakis?" and give them the opportunity to say "I'd rather you didn't"
Put your phone away
I know, you're just checking it real quick. But for the person who is trying to talk to you, it never seems as quick as it does for you. If you are having a conversation with someone it shouldn't even be in your hand. If you're not willing to tell the person in front of you that what's going on on your phone is more important/critical than they are put it away. If it's actually urgent tell them, deal with it, and put. it. away.
Send cards
It's always so great to get something nice in the physical mail, plus I do want the USPS to be solvent, so I advise you to send cards. Like, with envelopes and stamps.
Level One (Honestly even children should do this): Send thank you cards. You know the deal, you send these after job interviews, when someone gives you a gift, or helps you out in a major way. When you invited along to a dinner out, family vacation, skydiving expedition, etc and someone else pays for you, that's a gift and warrants a note. Sometimes, an email thank you is appropriate. If you don't even say thank you, you need to reevaluate your life.
Level Two (Impressive): Send birthday, sympathy, congratulation, and get well cards. When your parents call you and tell you random news about a family friend, old teacher, or neighbor, this is what you are supposed to do with that information.
Level three (Maximum): Send Holiday Cards. Nobody expects them from 23 year olds, that's why its so fun to send them.
Quit condescending to older people
I mean, this is basically "respect your elders" which I firmly believe (in most cases.) Someone who had experienced double the life you have so far by the time you were born might know some things you don't. Using a cutsey pet name for someone who was raising a child when you were still trying to figure out what you were supposed to use your feet for is arrogant and super dismissive. I also hate the whole "cute old lady/man" thing we throw at folks over 70. I have the utmost admiration for my grandmother, I'm not going to talk to/about her like I would a puppy. Actually, I'm a firm believer in not condescending to anyone, even kids.
Don't show up to parties empty handed
You don't need to roll up with pinterest-worthy baked goods, but a bottle of wine, a bag of chips, or some flowers is nice, depending on the type of gathering. Better yet, ask the host what they need. Sometimes you won't even have to buy anything, they might want an extra cooler or for you to bring that game you have that everybody likes.
Ask what you can do to help
When we were growing up, me and my brother and sister didn't learn that "please" and "thank you" are the pair that make up basic manners. There is also, just as importantly, "what can I do to help?" I distinctly remember my mom, when reminding us to be on our best behavior would repeat "please, thank you, what can I do to help?" Don't just sit there like a lump! Some people are particular about guests helping and will decline. I guarantee even these folks appreciate you asking. You also need to do this when you go back and visit family. When I'm at a friends parents and they lounge around while their mom runs around like a chicken with its head cut off to make us dinner I LOSE IT.
Be a gracious guest
If someone worked hard to make a meal for you, you need to try every part of it. I once served dinner to kids I was babysitting and the three year old asked me why I wasn't having any fish. When I told her I don't care for fish, she literally said "how can you know that if you didn't try it?" Owned by a toddler. Don't refuse food unless you're allergic or have a dietary restriction, which you should have made your host aware of well in advance. Bonus manners tip: try it before you start adding stuff. If you immediately slather food in sauce or butter or start going crazy with the salt it's like saying "I'm not going to even try this before I assume it's not good enough the way you served it." It's a lot of work to serve a nice meal, the least you can do is give the food a chance. (The second least you can do is offer to do some dishes)
Rein in the profanity
I like a well placed swear word as much as the next sassy twenty something blogger,and I have a soft spot in my heart for vulgar female comedians, though I will say my swearing rate fell off a cliff when I started working with kids all the time, and have occasionally been asked if I don't swear at all after replacing a few choice words with "frick" or "booty." Swearing all the time is a sure fire way to sound like a teenager. If you are swearing more than the people around you, you're swearing too much. Don't swear at work. If you know someone doesn't like to hear cussing, don't swear around them. Don't swear in front of other people's kids. If you're swearing in a place where kids are they hear you and some poor parent (or babysitter!) has to deal with it.
Dress presentably
I'm not going to slut or body shame you or tell you you should embrace my lifestyle of only wearing 100% cotton clothes that come from Old Navy. I want you to be comfortable and express yourself! But you should at least be clean and dressed appropriately for what you're doing/who you're seeing, if only to show people that you respect them enough to change out of your pajamas. "Get through my day without seeing any strangers butts" is a very reasonable desire that a lot of people have. We should respect it by only showing our butts to people who want to see them. Not that your butt isn't great, but margaritas are great too, doesn't mean I want one on the Orange Line at 8 AM.
Keep your hands to yourself
Don't touch people who don't want to be touched. Don't touch things that belong to other people that they don't want touched. Don't grub all up on that person that does want to be touched around folks who don't want to see it. There are many opinions on what kind of PDA is too far, I am of the opinion that in mixed company kisses that could be reasonably described as "pecks" are okay in limited quantities, as is hand holding, single arms around shoulders or waists and unsexy hugging. Basically I don't do anything to my boyfriend in front of family members that I wouldn't also do to a family member. It makes you look not only rude, but immature, to be getting down in public. You are a grown person who has full control of their hormones and faculties, show it.
Keep your hands to yourself
Don't touch people who don't want to be touched. Don't touch things that belong to other people that they don't want touched. Don't grub all up on that person that does want to be touched around folks who don't want to see it. There are many opinions on what kind of PDA is too far, I am of the opinion that in mixed company kisses that could be reasonably described as "pecks" are okay in limited quantities, as is hand holding, single arms around shoulders or waists and unsexy hugging. Basically I don't do anything to my boyfriend in front of family members that I wouldn't also do to a family member. It makes you look not only rude, but immature, to be getting down in public. You are a grown person who has full control of their hormones and faculties, show it.