Actually, that should be simpler than you think. :)

Just set the target attribute of your <form> to the name (or id in XHTML) of the target <frame>. That should make the data to be submitted there, and the original frame should not change at all, so it does indeed retain its data. Usually the problem is to get both to update, which makes you have to use javascript or posting to "_top" and redrawing the whole frameset.