sign up at their website. If you register by Monday, March 29th, they'll automatically credit your account with 20 free prints. You can redeem them until March 31st.
Previously, when they've offered the free 8x10's, shipping and handling is free if you choose in-store pickup - you may want to choose that option if you have one close by. Maybe a good time to get some pictures ready for a Mother's Day gift? An album for Grandma?