There are tons of different styles of sleeper sofas, so choose one that meets your needs. Check out some pictures:

This sleeper sofa looks just like...a regular sofa. That's the beauty of it: No one has to know it's a versatile piece, and you only have to unmake it when you need to.

This is a futon-style, which is the easiest thing in the world to deal with. Basically you push the back of the sofa forward until you hear a click, and then it extends so that the bed is flat. It doesn't have the typical sofa look of the picture above, but it's modern and very useful for someone who will be sleeping on it a lot.

This sleek design works exactly like the one above, except for the ultra-contemporary design. I love the bold red color and minimalist look.

Twin sleeper sofas are becoming more popular these days, because they can be a real space saver. If you don't have room for a rull sleeper, try this design -- or two of them pushed together, as in the pictures.
Pictures via Flikr