I've been coaching high school athletics for fourteen years. I coach indoor track(coed), girls' outdoor track and girls' volleyball. I think I have a total of perhaps 28 seasons worth of experience. Here is my improptu list of advice:
1.) At the outset of the season put forth some rules of teamwork, attendance and attitude. This primary set of expectations should be your bible for the rest of the season. DO NOT make any exceptions to these expectations. Parents may make excuses for attendance. Kids may have bad days, etc... It does not matter what the excuse, you do not want to bend. I say this not because I am an authoritarian coach! I say this because the biggest expectation kids have of their coaches is that they are fair. If you stick to your expectations the kids may get angry when it doesn't go their way but they will ALL love you for for even-handedness and fairness. By adhering to the same standards they will get their first taste of "team" and belonging before the first game or competition.
2.) Go into practices and competitons with a game plan. Get an idea of the drills, exercises etc.. and the amount of time they will take. Let the kids know in advance what they will be doing, how long they will be doing it and, most important, what it will do for them as an athlete and as a team. Giving meaning and direction to the practices is instructive in countless ways.
a. it teaches focus (how much longer do I have to keep this up?)
b. it gives small, achievable goals and teaches goal setting
c. it models good organization and competency for the kids
3.) Challenge the kids physically and emotionally. WARNING Do not be a drill instructor. Make sure that the challenge is commensurate with the skills/age of the kids. The best challenges are measurable and repeatable. Example:
run the mile and get each kid's time. Next week run it again (after practicing running and tellign them they are practicing for running). Let THEM write down their times because ownership is important. By the end of the season they will have measurable successes. This is only limited by your imagination as a coach and basic rules of safety

It is important that, at the end of the season the kids have a way of looking back and seeing how they have a.) improved athletically and b.) become a team.
4.) Be knowledgable! Get a book about the sport. Enlist the help of someone with specific knowledge. Ask questions!!! Kids and parents alike will adore you for your attention to detail, willingness to study and learn and your desire to bring the best experience to the kids and the team. As
kmsouthern said, "..kids are damn smart.." They want a coach to lead, to provide structure and to teach them something about life in which they have an immediate interest.
5.) Be schizophrenic! You have to wear many hats to be a coach. For some you are parent (because home isn't going so well), counselor, friend, coach or psychologist. Obviously, with different age groups the gravity of this changes.
No matter the age of the athletes though, all coaches have the need to play a variety of roles. The athletes must respect you as a leader and adult. They must love you as a mentor. Finally, they must fear you just a little bit.
I know that this last part is often met with debate but hear me out. A coach is in a unique relationship. One of your main suties is to have the athletes recognize their strengths and weaknesses and learn about themselves (and others) through the process. One of the best ways, in my opinion, that a coach can do this is by providing tough, but not impossible challenges to his/her athletes. In order to do this the coach becomes necessarily associated with the challenges they offer to their team. If kids don't occassionally say to themselves, "Oh man, what's coach gonna do if we mess up? How hard is practice going to be? etc..", then it is likely that you aren't challenging them enough. And as I always say (ok, so I just made it up but i'm keeping it) "challenge is the fertilizer, your team the garden" Also, realize that sometimes YOU may be impetus for the bonding of your team. (see #1) When the rules do not bend the kids will turn to eachother for support, advice and comraderie.
corollary to #1 and #5: you want your team to like eachother more than they like you..no matter how much that might hurt your feelings. They won't be growing up with you.....
6.) Have more fun than they are having
edited to fix only those typos I found...and I added my corollary