So true! Our daughters are 22 and 20 (she'll turn 21 on the first Husker home game, so that's going to be a crazy day!). I love the adult relationship that we have with the two of them. Thankfully we're the fun parents and they love spending time with us. Heck, their 20 something friends invite us out to the O Street Bars to hang out with them quite often (talk about feeling old!).
Boondock had some great advice here about walking the walk. The one thing I've learned is that your children are always watching and taking note, even when you don't think they are. What you do, how you act, how you treat others is really ingrained in them. Even more importantly, as father of 2 daughters, I always made it a purpose of mine to show them how a man should treat a woman, by how I treat my wife. I had great examples to show me (both sets of grandparents married over 60 years until parted by death, and my folks married over 50 years and still going). It's the best thing I can do for them, so they don't settle for less than they deserve.
The other piece of advice I would give it to just be there. You're going to screw up a lot as a parent along the way, but the fact that you were always there and always showed up for them when they needed it, that will stick with them forever.