I have found that flights look wide open. You show up at the gate and people crawl out of the woodwork from other cancellations or misconnects. Always have a back up plan. If you don't make the flight that your trying for, then plan via another city.

Carry a netbook/phone with internet access. You can easily relist and this prevents you from wasting time by trying to find an agent, because agents are already overwhelmed with assisting paying passengers.