I personally think that most of the parenting work has to be done before they reach the teen years. There’s a saying; “give me a child until he is seven and I will give you the man”. That’s not to say that you can leave teens to it without any input, but the foundations need to be there.
When they are younger, teach them about respect, being nice to others, trying their best at school etc. Keep an eye on screens. Young children don’t need iPads etc. Be there for them, and listen. On car journeys, in restaurants etc, talk to them. Don’t plonk them in front of a screen for an easy life. As they get older, teach them about the dangers of being online and raise potential problems in a child appropriate way. Teach them how to stay safe online, because there’ll be a time when they’ll have full access to it. Be aware of what they are accessing online (we had hard lines on certain games/apps etc even during the early secondary school years). In general conversation, bring up the dangers of drugs etc. Give them reasons for rules.
As teens, don’t be too strict. Does it matter if they want a few piercings? Does it matter if they want to wear make up? Does they really have to be home by an exact, arbitrary, time if their friends are allowed out half an hour later? Pick your battles. If they swear now and again, just rise above it. They have probably had a bad day and are venting at home.
Eating as a family as much as possible is good, because they talk.
Accept that teens are hormonal and may be a bit grumpy sometimes. Give them a hug and ask if you can help with anything. Tell them you know it’s hard sometimes. Explain that life isn’t always easy and straightforward and teach them how to overcome problems.