How do regulatory sequences in DNA allow the specific expression of lest say a protein in the mammary glands?

Lets say i was to create a transgenic animal, an animal which would secrete this protein in its milk. I would need to identify the promoter sequence that would allow the expression of this protein in the mammary glands. So, question is how would regulatory sequences work on the level of DNA to permit the specif expression? What technique could i use to isolate the regulatory sequence of the gene expresses specifically in the milk?How do regulatory sequences in DNA allow the specific expression of lest say a protein in the mammary glands?
Use a protein expression experiment or a protein database to find a short list of proteins that are highly expressed in mammary tissues compared to other tissues. Use the DAVID database (http://david.abcc.ncifcrf.gov/) to query these proteins and find which promoter(s) expresses the genes for these proteins. If the species of animal you are working with does not have it's genome mapped you will have a hard time finding this information.