Experience - you want to work with someone who has experience not just in PR but in the field and industry that your business is in. This way the PR agent will understand the competition you're up against, the media contacts to work with and the deadlines to work around.
Creativity - look for a PR consultant who has a flair for creativity - particularly if you're looking for PR on a budget. If you're on a limited budget you need to work with someone who can be quite creative and stretch your budget to the max.
Flexibility - how do you want to work? Some PR agencies work on a retainer where you pay a set fee each month for a certain number of hours work in return. You may prefer to work with a PR consultant who works on an hourly rate - and you may be able to cap the project so you know what you're paying out for. It's always worth asking this beforehand so you don't get any nasty surprises at the end of your project.
If you have any questions about how I work, please drop me an email at Julie@getinthepress.com or call me on 07827 290496.