There's a lot that can go right, and wrong, in an .htaccess file. That was just one snippet of a lot of stuff that probably should be in there to make it all work. The only other thing I can think is to make sure the line below is above the 2 lines I gave you before:
That line is part of the default .htaccess that comes with Joomla, but you may not have put the code above below that line (to enable the redirection in the first place). However, the .htaccess file can be disabled or restricted in many different ways not allowing the redirection I mentioned to work at all.
Additionally, if you're using a non-Apache server, something like Microsoft IIS or nginx, the .htaccess won't work. There are plenty of Joomla extensions that actually can perform the redirect for you automatically as well. I don't have any recommendations for this, but you can find them on the JED.. some are full SEF extension (like sh404) and others are just meant for this exact purpose.
Hope that helps give some pointers,
Alex