Custom Lookup field - how to display a value other than a primary key?

Hello,

CRM 2011: I have a custom entity lookup and when I select it I can access its attributes via Javascript/Json. The problem is the entity field on the form displays an id number (I presume it defaults to the primary key) after I select a value from the lookup. Is there any way I can change what value is displayed.

For example, the lookup view for my staff entity has ID, First Name, Surname, etc...but when I select a value the ID is left displayed on the form. What I would like is that a different entity attribute (firstname, surname, or combination) is displayed rather than the now meaningless ID.

Any ideas on how to achieve this?

Hope this is clear. Thanks in advance for suggestions.