Be fully present when you play with them. Really PLAY with them. Kids know when you are not paying attention.
Set firm boundaries. I once read that everything is new and novel for a child, so having firm routines and boundaries are comforting because they have a place that is safe and unchanging. You are not their friend, you are their parent. You might notice that people who try to be friends with their kids tend to spend little time with them...
Be a good and non-judgmental listener when they get older. They need you more at 14 than they do when they're 3, and the stakes are much higher.
Finally, be the person you want them to be. When my son was younger, I was obese, drinking, out of shape, whiney, unkempt and emotionally unstable. One day I realized that it didn't matter what I said, he watched who I was, and my words and actions didn't match up. I quit drinking, started weightlifting, went to a therapist. Funnily enough, these changes launched me out of a rut and into a much more fulfilling career. My son did model me, and now, when he struggles, I can talk to him about overcoming my challenges and it relates.
The most important thing to realize is your time with them is finite and you need to fiercely protect yourself against demands that take you away from giving them your full attention. You cannot comprehend how quickly your time with them at home goes by. You cannot reverse time. Your time is the only gift you have to give to your children.