If things are slow (and even if they are not), check on your licenses and see which ones are close to expiring (or recently expired). Contact those clients and see if they are interested in renewing or re-licensing some usage. After 9/11, when there was almost no new work out there, contact like that helped get a lot of photographers over the hump.
As your clients are looking for cost-effective ways to promote their products and services, your existing images have the value of having worked for them already! You may be exactly what they need right now. 🙂