Remove date from blog post URL

Hi,

How would I go about removing the date from the URL for a blog post? I've searched around and the solutions posted haven't worked for me. I'd like this URL:

http://www.tunepipe.com/jasonscheff/2015/03/09/is-you-found-the-one-the-best-wedding-song-ever/

to look like this:

http://www.tunepipe.com/jasonscheff/is-you-found-the-one-the-best-wedding-song-ever/

I've gone into Network Admin => Sites => All Sites and changed the permalink structure to '/%postname%/' but that just throws 404 errors. Any help would be much appreciated. Thanks!

- Scott