A cat I rescued in Portland last summer was pregnant. I knew what I was getting into with her, but I was still very nervous. I had been around cats and kittens, but never the birthing process. It is something I am so glad I witnessed! She did really well and I only had to help a little. I did have to have her spayed a few days after giving birth though due to an infection. It was scary because we weren't sure if she would continue to nurse the kittens or not, but it turned out ok too
My advice would be read everything you can and prepare for everything, even the worst. I had it arranged with my vet during one of her appointments that I could call her if I needed to, even at home. Not all vets will do that so yes, the number to a 24 hour clinic is a good thing to have.
I kept Sasha in my office so she could have quiet time. I fed her kitten chow (on the advice of my vet and many websites online) to keep her weight up. I had her nesting box all prepared about 2 weeks before she went into labor and had everything I might possibly need to help her nearby in the room.....towels, scissors, antiseptic, nasal aspirator, heating pad, etc.. The nasal aspirator (I don't know if that is the right name for it or not) came in so helpful because after her first kitten was born, the second one was coming before she could get the first one cleaned properly. It wasn't breathing and neither was the last kitten, which was born after a hard last labor. The third one was really big! I was able to save both them with that simple tool and a warm washcloth
Oh, and don't forget the camera! This is the link to the photos I took of that night and beyond. They are priceless and I look at them often. We kept her and 3 of the kittens and my mother in Cali took one.
http://www.flickr.com/photos/zoomommy/sets/72157601805667909/