The way I do this is to build a seperate form with unbound fields which I use as search criteria.
I then base my customer form on a query that uses the fields on the unbound form as criteria.
To reference a control on an unbound form you refer to the control directly:
For criteria, on a customer name you would first build a suitable unbound search control(text box or combo box) and reference the control in the forms query criteria
Forms!MySearchForm!MyUnboundCustomerSearchControl
Obviously your control name would be shorter but you get the idea.
Then all you have to do is get the form to requery every time the value changes in the text or combo box.
Hope that makes sense.