Using only %postname%
Anyways, your .htaccess file should be updating automatically. Did you change the permissions to 777, save the permalink changes, then change the permissions back?
If it doesn't update properly, WordPress will provide the code you need to put in the file. It appears below the permalink settings in a text box that appears. It says something along the lines of, "If your file was writable, we would update it automatically, but it's not. So here is the code you need to add: ____".