This has been the most succesful way in bringing in help. At the beginning of the year, I send home an interest form for parents who are willing to help. Then, I assign a (willing) parent on Tuesdays and Fridays for both classes. I've tried other ways of soliciting help, but this is great since parents can plan for their day in the room and the teacher knows when she has help coming.
I'm very lucky to have such great mommy-helpers!