Best way to deal with a changed url?

I have an app which lists events and uses slug generated from the event title for the url. Anyway, I noticed a couple of characters had been used in an event title which I'd forgotten to deal with and so amended my code and regenerated the slug for the event. That's all fine but now I've noticed a lot of traffic coming from google to the old url and people getting 404 errors.

So, what's the best way to redirect people to the new url? Is there something I can do in the .htaccess file to redirect? I tried adding a couple of Redirect 301 lines in the .htaccess file for the offending urls but it didn't seem to work (still got 404 errors).

Thanks in advance for any help..

Anyone??

mod_rewrite if Apache

Doesn't that just make it so the old url will work again? ..and not actually redirect? I'm wanting it to redirect so it can be indexed on search engines on the new url..