How do I empty the Drupal caches:
Effectively, how do you instruct an end user to clear his caches?
When you are logged as an admin (obviously, not every user of the site has to power to clear the cache), there should be a page in "Administer > Site Configuration > Performance".
And, at the bottom of the page, there should be a button (something like "Clear cached data") to clear the cache
As far as I remember, there's no need for Devel to do that, and you really don't need to go to the database, nor run some home-made PHP code.
As a reference, you can take a look at How to Clear Drupal Server-side cache