What systems are most effective for monitoring student attendance?
My institution does not monitor student attendance. That is my job. However, we have adopted Blackboard as our Course-Management System of choice across campus, and the student roster in Blackboard is updated every day by the Registrar's Office. We additionally have access to up-to-date course rosters through our implementation of the PeopleSoft productivity software. Presumably, the Registrar's office at your institution keeps up-to-date rosters and should be willing to share them with instructors. Otherwise, what's the point of having them?

There are two methods I use to enforce / verify attendance in a large lecture course, and both require comparison of my data with the roster in Blackboard or PeopleSoft. I could write a short program to sort through both files for matches, but I would rather do it manually to help me remember students' names.


Quizzes - I give short (and simple) quizzes randomly, but at least once a week. Fully half of the points on the quizzes are for showing up and putting your name on the quiz. For this reason, quizzes cannot be made up for any reason, but I will drop three or four of the lowest quiz grades. Since the quizzes aren't hard and the quiz average counts for at least 10% of their final grade, students want to be there for the quiz. I like this method because it allows me to do random point assessments of different topics and to start associating student names with performance. The grading is not unbearable, since i write quizzes that are simple enough to be graded at a rate of 10 seconds per student.
Clickers - If you use clickers, make each student buy their own clicker (About $40 US these days). At the beginning of class, they have to register their clicker with your software, which records the clicker's ID number. If you have a list matching clicker IDs to student names, then you take attendance every day automatically. It is easy to cheat this system, since a student can also bring their friend's clicker and sign-in for their friend also.


If you want to ensure attendance, I would use JeffE's approach: 


  Isn't the most effective way to ensure attendance is to make the lectures compelling and useful?