One thing that I did was to contact the local chapter of 'Moms of Multiples' club. I found out that they had a vendor night coming up and participated in that. I will also be advertising in their monthly newsletter and offering a 15% discount to all members for their first order with me.
Another thought that I had (haven't done it yet, but will next week) was to contact the Priest and office staff at my church. I'm going to offer to do wedding cakes at a discount for people who probably couldn't afford a full-price cake. This works for me 2 ways, 1) I get to help make someone's day special and of course, 2) it gets my name out there.
Get a magnet or decal made for your car. Make it look just like your business card & make sure if you have a website that the address is on there! Heck, splurge for one for each member of your family if they're willing to put it on their cars!
Ask your local library if you can post a flier on their bulletin boards. LOTS of stay-at-home moms take their kids in there for story times during the day! (I know I do! And I always read the bulletin board for anything interesting and new.)
Host your own bake sale instead of a garage sale or rummage sale. Make sure your business card or a label w/your info is attached to EVERYTHING.
Offer to teach a decorating class to a group of Brownies in your area. (Contact your local grade school and they can put you in touch with a troop leader) Prepare little goody bags for the girls in your class that include coupons for future orders.
Be a little pushy. By this, I mean that you should get into conversation with people as often as possible that you do birthday cakes. Make sure that you never leave your house without a couple of business cards in your pocket. Mention casually that you do cakes, when they say, 'Oh yeah?' you simply hand them your card that *just happens* to be in your pocket, and say, 'Yeah, give me a call for your next family birthday.' I hand out several cards every time I go to Wal-Mart because I get people to talk to me all the time.
Hope these ideas help you brainstorming! Good luck!