How do assign a form field element?

How do assign a form field element?

This should be simple but it's preventing the validation script from working properly. I'll explain:

Essentially, I have four form fields, all of which are prepopulated with a prompt ("Enter name here" sort of thing) which become blank on focus and repopulate on blur. Only two fields require validation which means the "Please enter your surname" prompt (Surname is not a required field) is being posted.

At the foot of the validation process I need scripting like this:

        if ($("#inf_field_LastName").val()=='Nachname'){
            ("#inf_field_LastName").val()='');
        }       
        if ($("#inf_custom_BabysBirthDate").val()=='Babys Geburtsdatum'){
            ("#inf_custom_BabysBirthDate").val()='');
        }

These two lines prevent the rest of the validation process from working. I assume the assignation is screwing things. (Sorry about the German!) Any ideas?

Grant