I send them a flyer and covering letter. Then give them a call a few days later. It also helps if you can find out the name of the Manager and address things to him.
If you use phrases like "I can ensure you and your clients continue to comply with all the relevant legislation with regard to the legal responsibility with rental properties".
This will usually provoke a call from them, because most haven't got a clue about the housing act, Landlord and Tenants act, and so on. Once they are aware you should get a bit of work. Also send them a copy of the ESC guide for landlords. It usually gets their brown juices flowing, if they are in the dark about things.
Cheers...........Howard