Ladies, I tell everyone that I know about
http://www.yourdoulabiz.com, which is a web-based program that I use to manage my doula business. Since I have about 50-60 birth clients a year, I'd be lost without it! In fact, my doula co-op group also uses it to manage our birth/pp doula clients, childbirth ed students and lactation consultations. There is even a free 30 day trial to see if the program is right for you...so why not give it a whirl?
I keep track of my on-call calendar at a glance, share contact info with my back-ups and invoice clients when they owe the remainder of my fee. I've gotten rid of my 3 ring binder with copies of paperwork and birth plans since I enter it all into YourDoulaBiz. And since contracts usually are signed and emailed to me, I keep them in my gmail account for future reference if ever needed, which is easy to email back to a client if asked for.