If you using Firefox
Edit -> preference -> security -> saved passwords -> Remove all
If you are using Google Crome
Tools > Clear Browsing Data - check all boxes and specify period "Everything".
There are several reasons why Google Chrome won't offer to remember your password:
1. You've previously told Chrome to never save you password for that site.
On Windows and Linux, you can resolve this by following the steps in the referenced help center article 
On Mac, there is currently no way to browse for and delete exceptions, so using the "Chrome Application Menu > Clear Browsing Data..." tool, selecting only the "Passwords" checkbox, selecting "Everything" for the time period, then clicking "Clear browsing data" should cause Chrome to re-prompt to save passwords for all sites. We realize this is not ideal and have an open bug to add a way to browse passwords and remove exceptions on Mac 
2. The site uses specifies that the browser should not remember passwords
Some sites, often banking sites (and Mint.com, @ddddod), specify that browsers should not remember passwords by marking the field with "autocomplete=off". For these sites, Chrome will not offer to save your password. We're considering adding an indicator so that you can more easily figure out which sites cause this issue. I've also heard of extensions that work around this issue, but that method is unsupported.
3. The site submits the login form in unusual ways