Consultant is a bad word to use: everyone now is an advisor, researcher, coach... But we prefer to call a spade a spade.
If you haven't commissioned or used a consultant (or any other professional advisor) before, then it can seem very difficult – you're paying for a high value service which is rather intangible. And you need to keep control of the process even though the reason you've hired them is because you don't know what the answer is.
Some of the things you need to think about are:
Be clear why you're using a consultant
the need for specific skills that are not in the organisation
the need for an extra pair of hands for a short period
the need for independence and getting an objective outside view
the need for fresh ideas
testing the feasibility of existing ideas
identifying a solution to a difficult problem
the possibility of training staff on the job by someone working with them
getting the job done faster
Are these good enough reasons not to try to do it yourself?
Write a Clear Brief
Consultants are not telepathic (at least not generally) and the clearer the brief the more focussed the work, and the less you will spend on things you don't need and disputes about what has been achieved.
Think about selection
It should be more than whether the consultant has done something roughly similar before. If you want an innovative solution then don't employ someone who has implemented standard packages for the last ten years.
Manage the process
Signing the contract isn't the end of the matter – make sure you know how you're going to keep contact and what resources you're going to put into making sure you have a successful conclusion
We've produced a guide for using consultants which goes into more detail. It's free to registered users and registration is free (see menu on the left)