The form of advertising that has worked best for me, so far, is just plain networking.  In bookkeeping, there is a lot of trust that needs to take place on both sides.  The best way to build trust, regardless of the industry, is to get to know who you are working with, and it is so much better to have someone that already knows you are trustworthy to say "yes, I can vouch for that person, once you talk with him/her you will see that he/she is trustworthy..."