If there are any strange cats coming to your house, you can try a deterrent to chase them away. This is one of the most common causes. Even if you don't see them and your cats are spraying near doors or windows, you can be sure you have visitors.
Clean sprayed areas with an enzyme cleaner. I've had some success in the past with calming collars. Be cautious of prescription meds. I had one cat that had seizures possibly caused by them.
Good luck! Also, if you haven't seen "My Cat From Hell", I would recommend watching some episodes of that. He has some good tips that may help your situation.
For me, though, it has just been attrition - getting my cat numbers down after the spayers have passed on. The spraying also decreased as they got older.