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..