If there is a specific person in your firm who looks after a client relationship, or is responsible for the vast majority of their work, you should mark them as the client owner. 

From the details tab of the contact's profile, you will see a section for the client team.
Clicking edit will allow you to choose a new client owner, or an existing one.

The client owner will flow through to all work created for that contact, allowing you to use work filters to show all work for that client owner in Work.

Further reading:
Adding a client manager