The National Preservation Companies usually have vendors whom do not just the trashouts. The vendor usually does the rekey/secure, initial and routine lawn maintenance and any work necessary to get the property in a saleable condition. If trashout and debris removal is all you do, you may try to get in contact with a vendor for the PP Company in your area and see if they can use your services for trashouts only.
I may be wrong but I dont think a national PP Company will sign you up to conduct debris removal only.