Come on, you know everybody uses pens! That's why you see so many of them imprinted with a company name and logo. Pens are inexpensive, and since people are always losing them, they are the perfect quick but lasting giveaway.
Choose brightly-colored pens with a real message to make them last in your customer's mind. Pens aren't the only writing instrument, either. Send out highlighters or markers to students, and they'll use them with thanks.