Do increase web server capacity when you know your ticketing system will get slammed. I have heard plenty of horror stories about what happened when the Royal Opera House releases tickets for, say, the Fall opera season.
Don't time out a user session when someone is trying to buy tickets! (This means you, San Francisco Symphony.) If you've got a hard session limit of 20 minutes set, configure that fancy system for which you're paying a lot of money to increase the timeout limit by five minutes every time there's an addition to the shopping cart. Someone trying to buy 2 tickets to 15 concerts, with seats in different parts of the house, can easily hit a hard time limit. You do want to sell those tickets, right?
And a small additional word about choose-your-own-seat software: if Brown Paper Tickets can do it for the El Cerrito High School theater, where Berkeley Opera performs, your organization can figure out a way to do it too. Okay, if you're a small chorus with open seating for your concerts, this is not a big deal. But if you're LA Opera or a small theater company, it is a big deal.