Scott, I've owned my marketing agency since 2009 and 2 ways I've found to get people over the hump, and become their trusted advisor, are:

1 - Offer a free or reduced cost trial. Note: Make sure the client can afford your standard pricing and that they understand the 'trial' is a one-time event.

2 - Educate them (my preferred method). I typically do this by showing them how they can achieve the results they want by doing it themselves. Once they understand what is involved, and that you are the expert, they will want to hire you. Very rarely does a qualified prospect decide to try it on their own.

I hope this helps.
Ed