No pun intended.
I’ve had a number of prospective clients who’ve had a negative experience working with a design firm or marketing company in the past. Their stories include lack of attentiveness, missing deadlines, too many mistakes, over-promising, unsuitable ideas, or simply delivering uninspiring creative. The situation that seems to make it most difficult for prospects to ‘trust again’ is when the previous team failed to get the basics about what the company does, and their brand character. The result: they’re gun-shy on committing again.
How can you know it’ll work this time? 
Learn from bad experiences. Define what didn’t work as best you can and look for that when scouting out potential new firms. Make up a specific list of questions designed to uncover that ‘flaw’ and even consider talking openly about it to them, so they know what not to do.
Make sure you feel comfortable with them in every respect. You should be able to have a couple of conversations before any commitment is made. These kinds of discovery sessions are needed to write a proper proposal anyway, so maybe you ask for a little more time or a second or third meeting. Creative types may be different from other service firms, but this is about business – your business, so don’t feel you have to accept a flakey demeanor.
Look past the showy stuff. Request client samples that are most relevant to your company. Ask for details on what the purpose was, the goals and the outcome, and why they consider it relevant. Judge if you think it’s good work given the rationale and make sure there’s a variety of work and styles (not all looking the same). This should provide confidence that the firm is creative, diverse and understands your kind of company and your target.
You don’t have to buy the whole cow right away…
If they don’t offer a deposit system, request one and determine points of amiably ending a project if not going well, or a kill fee. For example, a simple corporate identity package with a 50% deposit required: if the first round of concepts presented are way off the mark, you could agree to end the project there. The firm should be covered for work done-to-date and you’ve minimized your money wasted.
I’ve had a prospect ask me to break down projects even more, and sometimes it’s just not possible. Using a simple corporate id again, the first and largest chunk of work is one exercise and actually worth more than half the budget.
For more complex proposals, if the firm has put forth a number of items, ask what is the least you need to commit to in order to move forward.
See it from our side.
It’s not fair to either side to ask firms to speculate on what they might do for you (i.e. types of logo design) until you actually engage them. This takes real work on our part, so if we did try to answer off-the-cuff it would likely not be a good answer without the thought and research needed to do it well.
Unfortunately, doing work on spec (I’ll pay your for it if I like it) is also not ethical. I know some firms do it to win clients, but the practice is bad for the entire industry. They don’t let you leave a gas station without paying for your fill-up and come back later to pay you if you like how it drives. This is where the retainer with agreed ending points or kill fees comes in handy.
Take the leap!
I have changed the way we usually do things and gone out of my way to make prospects feel comfortable with the decision, and we’ve never let them down. I am willing to bend over backwards because I’m personally driven to right the wrongs done by bad designers. However, there’s only so much time we can spend and efforts we can make to win your trust if you’re just not ready. Take your time, but eventually you just have to take the leap and let us try.