Scott Allen has a very good blog post which he calls “Master Pages: Tips, Tricks, and Traps”. Another way to describe it might be “Avoiding getting into trouble with Master Pages, and how to get out of it when you do get into it.”
He delves pretty deep into not only what Master Pages are, but how they work, which can help you fill in the gaps when you do get into trouble.
As he says:
Master pages are a great addition to the ASP.NET feature set. Master pages help us build consistent and maintainable user interfaces. Master pages, however, are not without their quirks. Sometimes master page behavior is surprising, and indeed the very name master page can be a bit misleading. In this article, we are going to examine some of the common problems developers run into when using master pages, and demonstrate some practical advice for making effective use of master pages. For an introduction to master pages, see "Master Pages In ASP.NET 2.0".
To make use of master pages, we first need to understand how master pages work. Many of the tips and traps covered later in this article revolve around understanding the magic behind master pages. Let’s dig into these implementation details first.