One mistake that people seem to make repeatedly is to grossly underestimate the time it takes to find a new job. The majority of people take the attitude of "Hey, I'm going to enjoy my severance for a while and then later I'll think about getting a new job." Time flies when you're not working, and before you know it, time (and money) is up.
Here's a sobering rule of thumb: Career coach and author Katy Piotrowski says that it takes, in general, a month for every $10,000 you want to make. So if you're looking for a $50,000 job, you can plan on it taking five months--way longer than most severance packages.
Katy does have some good news in her forthcoming book, The Career Coward's Guide to Job Searching (which went to the printer yesterday): If you spend more than an hour a day working on your job search, you can shorten the time it takes to find a job. She recommends devoting six hours a day to your search (obviously, you can't do this when you've still got a job). In this way, you can shave months off your search time.
Meanwhile, in this tough economy, I think all bets are off. So, as they said in the Little House books, make hay while the sun shines.