When is it a good idea to get a second cat?

I have a nearly 4 year old neutered female cat. I have been thinking about adopting another cat, but I fear they won't get along. Current cat is not really social, rarely likes being touched, instead requires attention: playtime, being near (but far enough), etc.

What are your experiences with second cats? Are there age / gender combinations which are the safest?

Edit: thanks for all the replies! Forgot to mention that she is an indoor cat.