Prevent User From Skipping Records In Continuous Form During Data Entry

I have a main form with several continuous subforms. Each subform consists of several listbox controls. I would like to require the user to select an item from the listbox before being allowed to move to the next record in the subform, and upon reaching the last record in that continuous subform, to require an entry there in order to move to the first record in the next cont. subform.

As an added bonus, it would be nice, though not necessary, to automatically jump the focus from one record to the next after data is entered. But my basic goal is to avoid skipping records.

Caveat: I cannot use the "required" option in the field to which the control is bound because that field has a default value previously entered using an append query. (The default value basically means "not yet entered" and is not one of the options in the listbox. I am using this because this field is a foreign key in the table, thus it must have a value in order to have a record with which to populate the subform.)

My apologies if this has been asked and answered elsewhere, but I've searched and, while I found a few related threads, they don't quite answer my question. Here they are for reference:

http://www.access-programmers.co.uk/forums/showthread.php?t=38364
http://www.access-programmers.co.uk/forums/showthread.php?t=69047
http://www.access-programmers.co.uk/forums/showthread.php?t=103176

Thanks for any help! Also, if you are going to recommend VB code, could you be specific about how and where I should use it? I'm not afraid of code, but I'm definitely new to it...