One professor claims you do it by criticizing the Bush administration. This story has been going around for a few days, and I thought I'd collect a few more details. (I originally found this through volokh, which has a number of posts on the topic.)
The story is that a Princeton professor tried to fly somewhere, and was flagged for screening. When he asked why, the person at the counter asked if he had been in any peace marches, and he said no but he had given a speech critical of Bush. The employee said, "That'll do it." On the way back he didn't have any extra screening but his luggage didn't arrive until the middle of the night.
The professor claims that he was put on the watch list to punish him for his speech, and he's received a good deal of support from various blogs (as well as some skepticism). I think when it comes down to it, if someone said "Bush ate my kittens with his bare hands" some people will believe it no-questions-asked, because you know, Bush is the kitten-eating type. But let's think about this in detail, and let's even assume that the administration is willing to pursue this sort of retribution. They decide to punish dissidents, and they pick out a professor that no one outside his field has really heard of, because Cindy Sheehan, Jon Stewart, and Howard Dean would be too obvious. Then they let word pass down to random ticket agents at American Airlines that they put people on the list for peace marches and speeches. They don't harass him on the way back though - because they have something better planned for him. They're going to instruct the baggage handlers to lose his luggage! Muwahahahaha. Well, for a few hours anyway; they'll deliver it to his house overnight.
Seriously? We're supposed to believe this, because of what this guy says a ticket agent told him when he was selected for extra screening? Occam's razor, anyone? The moral of the story: the terrorist watch list is really annoying. It should be handled much better, if it all. However, you lose all your credibility to address the actual problems if you complain about situations like these that are very likely exaggerations. Show a little skepticism, people!
(An article in Slate a while back did a good job explaining the watch list. He came to what I think is a wacky conclusion - "Let's suppose—just suppose—that the No-Fly List has caused only one terrorist not to board an airplane with a sharp tool or explosive shoes. Wouldn't that still be worth these mild inconveniences? Of course it would." To which many people have replied with something to the effect, "What else should we be willing to do to prevent just one terrorist from boarding a plane? Fly naked? Every time we are asked to remove our shoes at the airport, we should be thankful that Richard Reid wasn't known as the underwear bomber.")