I'm in the same phase...trained, trying to certify, taking on clients (for free right now). My business cards list my name, contact information and business name on the front with "A birth doula service" under the business name. The back has a picture, a kind-of motto (maybe? LOL), and a breif descriptive list of what I do as a doula since many in my area are unaware of what a doula even is at this point. On my website I state that I'm a "DONA trained birth doula." I don't feel that it is required for me to become certified, but I'd like to do so for my sake. People ask if I've been to a class, but that's about it. My trainer said she'd only been asked maybe once if she was certified and she's been a doula for years. It probably depends somewhat on your area, the birth professionals you work with, and your clientell as to whether it is worthwhile to certify or not.
Right now I'm "advertising" mostly by word of mouth. In my small town, if I get a few births in and (hopefully!) help some families have very positive birth experiences, once they start spreading the word I'll be set. The local CNMs (who work at the only OB practice in town) said they'd give out business cards if I gave some to them, so I've done that. I don't know if they hand them out in the "welcome to pregnancy" type of packet or not, but either way, it's good. The local CBE also added my information to her resources list and I know from atleast one person that they did talk about doulas in the class. I figure I'll advertise eventually, but not untill I'm certified (or atleast feel sufficiently experienced to start charging for my services). I probably will do a booth with handouts, information, business cards, etc at a local fair in August just to help spread the word.
Best of luck!