First thing I have to say is the advice on the forum is generally great, but one thing that isn't anywhere near stressed enough is the importance of knowing your market. A lot of professions, notably Lawyers, medical professionals and Realtors, have very specific rules about what they can and can not do in advertising. These rules vary from state to state, and failure to abide by them can cost a business its license to practice in that state.
Having a good idea is not enough, you have to know exactly how to implement it for a certain business. To give you an example, a law firm in South Carolina can not have any testimonial unless that testimonial is verifiable fact, recorded on a sworn affidavit, taken by a lawyer working for a firm with no financial ties to the firm for which the affidavit was taken.
Why is this important for you the marketer to know?
Simple, if you want to establish credibility with me don't tell me how badly I need a testimonials section on my website. Yes, that is great, general marketing advice. It would also cost the Lawyers in the firm their law licenses. More importantly, to you, it costs you any authority you might have as someone who understands my market.
If you want to establish credibility in a market here are some important steps you can't overlook.
1) Find out if the profession has any special licensing requirements in your state.
2) If it is regulated, find out what licensing body oversees regulation.
3) Call that licensing body and ask where you can find a copy of their Rules of Professional Conduct (or similar name).
4) Learn them backwards and forwards, making sure you find out what any language you are unclear on means.
Sometimes you will find out that your great idea is in violation of the rules, or that the pay structure you want is a real no no. Regulated professions often have very tight rules about giving % commissions.
Trust me when I say that businesses will love you for knowing their particular circumstance. It is how a sales pitch I made for hosting and SEO services resulted in a full time job offer.