This sample db demonstrates user setting of required fields at data entry stage.

The field list is built up, based upon the record source for the form.

If the FieldList table is empty, it gets populated automatically, on opening of form F_DataMain.

For fields to be pinpointed as mandatory, the adjoining check box should be 
selected (in the list displayed on Set Required Fields tab).

During data entry, form's Before Update event checks whether any of such 
mandatory fields is blank. If so, suitable warning message, listing such blank fields, 
is displayed and update action gets cancelled.
 
Versions - Access XP & 2K (In Access 2000 File Format)
References - DAO 3.6

Download Access XP & 2K:  http://www.rogersaccesslibrary.com/forum/uploads/37/FormSetReqdFields.zip - uploads/37/FormSetReqdFields.zip