A personal best is always a major victory:There's more, click through.
It doesn't matter if they finish first, third, ninth, thirty-eigth, or dead last. If they swam the event faster than they've ever swum the event before, it's a victory. This is still true if they've never swum it before.
Cheer for your children:
Do not yell at them. Do not tell them that they're swimming poorly. Never, ever, ever ask them what the hell they thought they were doing, particularly in the first ten seconds after they get out of the water. You're paying good money to put them on a swim team that has actual coaches who can handle all of the criticism (and who know more about how to swim and how to coach than you do). You're there to encourage them, not discourage them.
Cheer for other people's children:
If you've got a pair of lungs that can rupture eardrums at fifty feet, why is it that I only hear you during a few heats? Your kid is on a team. Support the team. If you don't know anyone who is swimming in a heat, cheer for everyone. It's a hard sport, and a little support makes everyone feel better.
Be a role-model for sportsmanship:
And when I say that, I'm talking about the good kind of role-model. Most swim meets are like most cereal box contests: many will enter, few will win. Your kids are going to get a lot of practice at not winning events. Teach them to show as much grace and class when they don't win that they do when they win.