Using mod rewrite to force people to use "www" in the url

[Brother Malachi]

I'm trying to force an auto redirect on those that don't use "www" in their url. Does anyone know how to accomplish this using .htaccess and mod rewrite?

The reason being that vb uses cookies to store login data. So if I goto http://www.myforum.com and login, then goto http://myforum.com I will still be logged out since the cookies stored were for the "www" domain. This becomes a huge issue with several mods that use hand written links and not dynamically generated. (I.e. instead of using the current address (meaning, it will use "www" in the url if the current url has it), they use a hand written one...regardless of whether the link has "www" in it or not. I.E. one of the mods inserted a link into my navbar that is always without a "www". Therefore, users often login with "www" in the url, then click on the link and are logged out.