There is no way to consistently build relationships with every multifamily broker in every market you cover without systematizing the process using customer relationship management (CRM) software.
The process in theory is easy, but carrying out the process does require a lot of work. This is a good thing, it ensures few will do it right for a long period of time but those that do earn tremendous success in their investment careers.
Buy a CRM license. There are lots of them out there. I use and love RealNex (http://info.realnex.com/lead_demo-0). This will be the platform that you do all your networking with brokers on.
Search for every multifamily broker in every market you cover and save their contact info into the CRM.
Save their contact info to your CRM.
Schedule regular, out of the blue calls.
Make introductory calls for new contacts.
As you continue to make calls and develop relationships, learn about them.
Watch the video, read the details in the video description for more specific details.