Not everyone is comfortable with mucking around with 301 redirects and .htaccess files, but redirecting from yourlovelysite.com to www.yourlovelysite.com is pretty darned important (or vice-versa). If your lovely site is a WordPress blog, then you don't ever have to get your hands dirty playing in the 301 cesspool again.
There's a stupid-simple WordPress plugin called Enforce www. Preference that does all the work for you, and it even goes one better...it also will strip off index.php from the ends of URIs.
I just tried it over on my Dazzlindonna blog (because I'd forgotten to deal with the www issue there), and it couldn't have been any easier.
- Upload the plugin.
- Activate it.
See? Stupid simple.
If you've used the www in the url of the Options / General section, it will redirect non-www to www. If you haven't used www in the Options, then it will redirect www to non-www. So it just uses whatever you've already decided upon when you originally installed WordPress.