Stop promoting on webmaster forums like most other owners. Start advertising on teen forums, forums related to facebook etc. That's what I did with ProxB and it has been one of the most succesfull proxy lists from 'low-earning owners' in just about 2 months.
Another thing is that you should redirect your proxies to the list before you remove them as proxies. I remember 200 proxy domains expiring within 2 weeks, I just redirected them all to ProxB and got like 25% returning visitors from those