First, the terrorists may not have been as coordinated as many people suppose. They didn't take any steps to insure that the attacks would happen at the same time, AFAIK they didn't even have some coordination to insure that all of the flights where in the air before launching the attack. If they had the two planes in NY circle the city once or twice before slamming into the WTC, then they could have had all of the attacks happen in short order. This would have prevented the PA flight from being stopped.

Second, while the terrorists where well hidden in the Middle East, you also have to remember that over there they are on their home territory. The terrorists simply may not have realized that every credit card used to buy a ticket for the hijacked planes would be traced within a few hours of the attack. They had years, if not decades, of experience in not being traced in Afghanistan, but Afghanistan is not an information society.

Third, avoiding retribution may not have been one of their goals. Terrorists sometimes act with the goal of triggering a massive retaliation, which justifies and provokes even more extreme acts of terrorism. In some ways, being associated with the worst attack on the US is an advantage for bin Ladin, it gives him massive credibility and moral authority.

Fourth, some of the people being caught easily where probably not considered part of the group by those making the attacks. The people that acted as runners and go-betweens may not have been told who they where working for, they where simply paid to carry a message from person A to person B. The terrorists would have no concern for covering for these people.

Finally, the terrorists that where actually making the attack probably where not in the best of mind sets in the days right before the attack. It's very likely that in the run up to the final attack they would have gotten a bit sloppy in covering up traces of their actions.

Jay