This is a very simple tip and a rule that you should live by MOST of the time. Of course a situation always arises when you need to do something that is not recommend, but in general this rule should be followed:
When to Use XML Web Services
Application to Application Communication
When not to Use XML Web Services
Intra-Application Communication
That's it, that's the tip. It may seem obvious, but I've seen XML Web Services used in the wrong way more than once so I know it's not!
Until next time!
More Information on Web Services:
http://msdn.microsoft.com/webservices/default.aspx
http://www.dotnetjunkies.com/quickstart/aspplus/
http://www.dotnetjunkies.com/tutorial/default.aspx?tid=6
http://www.dotnetjunkies.com/Search/?exSearchPhrase=Web%20Services