I've been a doula for six years now. For the last three I've been practicing as a midwife as well, so that has added something to my resume that a lot of other doulas don't have. Even before that, though, I was getting hired about 80-90% of the time and felt like I had the work I wanted. We have many, many doulas here, though there are only maybe 10 or so who cover my area and make it their main job.
Some of the things I did that seemed to help: I stopped bringing packets to my consults -- many women will still want that stuff later, but I found that my hire rate went up when I stopped giving out *anything* at the consults. People are drawing conclusions about you on very little information and if you are flexible about the role that you are going to perform then backing off on the "doula as educator/advocate/fighter" impression might help you. I don't know how to be articulate about what I'm trying to say here, but sometimes women feel a little bit unsure about taking on any battles when they are pregnant or about making big changes and presenting yourself as the person who will change their birth experience might be a turnoff.
I also stopped bringing my "doula bag" to consults. I used to bring this big duffel and show them all the stuff I would use to help them at their birth -- all the standard doula toys. I don't carry that kind of stuff at all any more. My doula bag is a small tote with copious snacks, canned coffee, knitting, and a toothbrush. I just found that most women didn't want/need any of the stuff that I was bringing. They just wanted someone who knew what was going on to help them and make sure that they didn't get run over.
Marketing myself directly to women never worked for me, except in one way: I got on as many internet listings as possible. I also found it helpful to offer to visit natural childbirth classes as a guest speaker and talk about doulas.
One thing that worked that was a surprise to me: I asked other local doulas to back me up. Sometimes there would be a small exchange of money, but often we would barter and what I found was that all of the doulas who were backing me up would ask me to back them up. In addition, they would often think of me first when they were booked. And the doulas around me get booked up really frequently. A lot of them are not really doing this full-time and so they will turn away a reasonable number of people once they get their one client a month or whatever.
If I had a provider who was at all decent or accomodating, I would send them a thank you note with some business cards. This has proved really fruitful for me because some women will ask about doulas at their appointments.
Lastly, consider raising your fee. Having a high fee, as long as you have a couple of years' experience under your belt, creates the impression that you are more experienced or more skilled than the rest of the pack. I know it sounds crazy, but I watched the midwife who trained me charge about $300 more than anyone else in town and get hired consistently by lots of women who thought she had some kind of special doula magic.