With some of our businesses having over 10,000 clients on record, it is important to manage clients properly.
Under Settings -> Interface, you should set Client Selection to "Search by Typing" which will show a search box everywhere you need to select a client. Once you reach around 150 clients, it becomes more efficient than displaying all clients in a dropdown select box. You can type any part of the clients' name, email address, or id, to have matching clients displayed instantly.
Clients that hasn't been issued any documents can be deleted. As soon as a client has been issued something, he can no longer be deleted, as we allow clients to login and view all their generated documents. Clients like these can be marked as inactive though by unchecking the Active checkbox when editing the client, or viewing the Clients page. These inactive clients won't show up in searches or on any place where you need to select a client.
Clients' active status can be restored from the Clients page. First check the Include Inactive Clients checkbox at the top of the page, and then use the page as you normally would.