Any tips on integrating a new cat into a one cat household?

I was wondering if anyone had any tips,

My original cat is 14 and very mellow. She doesn't really interact well with big groups of cats, but has been on good terms with other cats before.

The new cat is much younger 3 or 4, she is pretty mellow too but really likes exploring.

I've read a bunch online about having a safe room for the new cat. I was wondering if you all had any super awesome and helpful tips for me.

Thanks!

Update: it's been about four or five months now. The cats are doing great. They don't fight or anything. Usually they won't play too much together. But they hang out by eachother all day. http://i.imgur.com/rbigS.jpg