I want to add a button on edit page of a record (say button which opens an external URL in a new window, may be a a url for reference). How do I achieve so?

There's always option of overriding with visualforce page, but there's lot of pitfalls around that approach (say a new recordtype or field is added, you need to create new visualforcce page or update the page respectively etc)

Any idea on how to achieve this? 

Chirag