This post by Scott Hanselman is just a brilliant starting point for beginners in application architecture (in any language actually). In my experience a good basic structure can solve so many problems in an application - and in many cases is a 'freebie', i.e., it doesn't really add any more development time. He does make an interesting point about DataSets in the Business layer...which I'd tend to go along with in the most part - if however you start to get objects which almost exactly replicate the structure of your DataSet...the benefits can be non-obvious - but decoupling your layers by having no reference to underlying back-end stores is almost always a good thing (unless you're doing a DB admin app of course :-))