For several years I had my 3rd graders learn and recite the Preamble as part of their yearly curriculum. When I moved back up to 5th grade, most of the students remembered it from 3rd grade, so it is a great opportunity for students to memorize a great piece of history!
The best way to practice the Preamble is to download "We the People" from SchoolHouse Rock on iTunes, and play it several times for a couple weeks. The kids catch on quickly, and actually, it is how I memorized the Preamble to