If your classroom is large enough or your class size small enough, arrange your desks in a partial circle, facing in toward the center of the classroom. When the teacher is positioned in the middle of the circle, each student will be facing the teacher and spaced an equal distance from her. This allows close eye contact between teacher and students and a stimulating, conversational environment where it's easy for the teacher to keep an eye on all students and keep them engaged in the classroom activities. For a variant on this setup for larger classrooms, create two rows of partial circles, but require students to alternate sitting in the front and back rows every other day.
If your classroom is large and arranged in traditional rows, take advantage of the front row for students who need a little extra help in focusing, such as students who have been discipline problems, students who tend to be quiet and uninvolved, and students with attention deficit hyperactivity disorder (ADHD). Use the rows behind for students who have an easier time keeping themselves focused and engaged so that you can keep those needing more attention engaged more effectively in the front row.
If you position the desks in your classroom so that students won't have major distractions in their sight lines, you'll have an advantage in teaching. Though sunlight and fresh air are good for a learning environment, you may want the desks facing away from windows so that students can't watch what's going on outside (or just close the blinds). Things, like class pets, computers with screensavers or anything else that moves, should also be kept to the back of the room.
With a large class, arrange the seating in your classroom so that you can walk around between the aisles of students when you're lecturing. This way, every student will have incentive to stay focused on what you're saying as you walk by and make close eye contact or view their work and behavior.
If your classroom is dedicated to a particular subject, consider arranging the desks and chairs according to the classroom experience for that subject. For example, if teaching a reading class, you may prefer to arrange the chairs in small clusters, facing each other, for group discussions.