How to return XML in ASP.NET?

I have encountered many half-solutions to the task of returning XML in ASP.NET. I don’t want to blindly copy & paste some code that happens to work most of the time, though; I want the right code, and I want to know why it’s right. I want criticism; I want information; I want knowledge; I want understanding.

Reading XML with an “&” into C# XMLDocument Object

I have inherited a poorly written web application that seems to have errors when it tries to read in an xml document stored in the database that has an “&” in it. For example there will be a tag with the contents: “Prepaid & Charge”. Is there some secret simple thing to do to have it not get an error parsing that character, or am I missing something obvious?