I have two sons, one of 24 and one of 13.
My two pennorth, for what it's worth:
Listen to them. Treat their opinions and thoughts with as much consideration as you would an adult's.
Tell them you love them. Show them you love them. Tell them that you always have their back. Show them this too.
Demonstrate good manners- show respect and consideration for others, and insist that they do too.
Discipline them when they need it (I don't mean smacking or anything, obviously)
Laugh with them.
Have firm boundaries in place- whilst their needs always come first, their wants don't always. Show them that their parents are allowed time to themselves/to have things when they don't get them.
If they're boisterous, take them outside for a run. Play football or whatever their 'thing' is. Introduce them to your 'thing'.
I had a rule that if I paid for an extra-curricular activity then it had to be followed through for at least half a term. Other than that, don't force them to continue with football/karate/country dancing if they dislike it.
Teach them to tidy away their things (but I warn you, this may not 'stick'!
)
Just be interested in them.