Yap has some support for automatically refreshing the document, but it is not at all perfect. The two most annoying problems:
The Sumatra PDF viewer (at version 0.9.1 at the moment) is much much better at this. The file is refreshed without any user interaction an so far it has not happened to me that it locked the PDF and prevented pdftex from overwriting it.
Please, if possible, make Yap work just as well for DVI files as Sumatra works for PDFs.