Problem: The restaurant is fully booked.
Solution: Put yourself on a waiting list. Don't be afraid to emphasize how much you want that table, as your eagerness may help persuade the staff to fit you in, even when they're booked.
Problem: You hate your table.
Solution: Speak up quickly. Most hostesses say they prefer to know before a diner has been seated. Of course, you yourself may have no way of knowing if you're happy until the hostess walks away. In that case, don't waste your time complaining to the waiter. They don't decide where you sit.