I found it usefull to give every ASPX page its own CSS class name. The codepart (from my master-page) looks like this:
<%@ Master Language="C#" AutoEventWireup="true" CodeFile="Site.master.cs" Inherits="Site" %> <html class="<%=Page.GetType().Name%>"> ... </html>
It is not 100% nice (designing a CSS sheet can be very rewarding), but most of the time I’m is busy creating a web application. This way I can quickly add CSS, without breaking other parts of the web application.