Well, I am in a bit of a hurry, so if you are not aware of what asynchronous web-methods in ASP.NET web-services are, or why and how to use them, check out this article on MSDN.
Essentially, you split your regular web-method (let's say XXX) to a pair of web-methods (BeginXXX and EndXXX) with particular signatures, enabling them to run in background freeing-up the ASP.NET thread pool thread for handling requests. This is only useful if your web-method is expected to perform a long-running task.