Hiring a creative agency or freelancer to help your company with design can be a daunting task. There are a lot of factors to consider and finding the right personality and process match in an someone can be difficult. Openly and honestly discuss your company and goals for the future, and who you are as a leader are crucial to finding the best pairing.
Put aside your preconceived notions
A designer's job is to take your story, dreams, hopes, ideas and transform them into a work that represents who you are as a company. Let them do what they do best and translate those thoughts into something awesome. You'll be much happier with the final product. You'll likely be surprised with the angles you never thought about.
Don't ask for spec work
You don't go to a hair salon or barber and ask them to cut your hair and only offer to pay if you like it. Don't ask an agency or designer to do the same for you. Expect your request to be rejected outright. If you need to insist on exploratory designs, pay the designer their going hourly rate.
Be prepared to talk about money
The bottom line for a designer is that they need to put food on the table, for them and their family. Go into preliminary discussions with an idea of how much you're willing to spend on hiring someone. Also consider who you're hiring. Agency's will cost more, but you're hiring a hive mind to research, design and develop. Freelance designers may be less expensive, but no less qualified.
Going into negotiations with your budget helps you and the designer establish what parts of the project will fit into your budget. Without the knowledge of what you're ready to spend, plan for a longer negotiation and project exploration process.
Judge someone's abilities by what they've put into the world
All designers have good intentions to create awesome; to design or build something that will give great acclaim. Judge a designer or agency on the fruits of the previous labour. Look over their portfolio and see if the works they've released to the world fit your fancy and business model.
I believe that most of this is just using common sense. Going into meetings and negotiations with an open mind, prepared to listen and think critically, will go a long way to finding the right fit in a designer.
Go forth and find someone awesome.