Disable browser autocomplete feature on form fields.
Modern browsers are getting quite persistant when it comes to automatically filling in form fields with username/password combination they feel to be the right ones. Most of the time that works fine, but if it doesn't then it's a pain.
The html option autocomplete=off was/is meant exactly for this purpose, but is completely ignored nowadays.
These 2 solutions do work in most situations. The first one worked fine in the past, but browsers are getting a bit too smart for that one. The second one works like a charm as far as I can tell, and is quite elegant as well: