Avoid those small agencies where the PR is just a bolt-on to other marketing services. In practice, this can mean that the proprietor is actually an expert in desktop publishing, logo design or internet marketing but does a bit of PR on the side. You wouldn’t hire a decorator to install a new gas boiler in your home, after all, so why hire a logo designer to do PR? They may not realise that public relations needs skills and knowledge, but need them it does. A small agency that will be good at PR is one that specialises in it.
Moreover, if you’ve got a relatively small budget, a specialist PR agency that’s already proposing ideas to the publications and websites that matter to you will find doing your work easy. If they’re just doing PR as a bolt-on to other services, they may have to develop trust and relationships with media outlets from scratch.