You could ask schools if you can put a used crayon bin in the lobby or in classrooms, next to the Labels for Education bin. You decorate the donation box so it's attractive, then go in to collect the crayons on occasion. You can pay the school a modest price per pound. Schools are already collecting labels, Capri Sun packages, and possibly other items for Terra-cycle, so putting in a crayon bin is no big stretch.
You could try placing your donation bins in teacher supply or office supply stores, too. Really, anywhere people with kids or who teach go frequently and repeatedly (if it's somewhere a person goes only once and a while, they'll never remember to take along the old crayons to drop off).
If there are child-centric businesses or organizations in your area that have Facebook pages, you could post on their walls. Think children's museums, Chuck E. Cheese, etc.