Most people hire patent attorneys when applying for a patent, but no one really talks about how it all works and the things you should consider when hiring one (or a whole firm for that matter). I think hiring an attorney is like finding a new hair dresser. You should feel comfortable and be able to communicate freely. Aside from having natural chemistry, your attorney, like your hair dresser, should understand your goals (i.e., what you want to do with your invention vs. what you want your hair to look like) and gain your trust (i.e., handle drafting, filing, and prosecution on your behalf vs. you're at mercy of whoever is cutting your hair).
The first thing you should consider is your budget. If you're a start up and you're tight on budget, you probably should not go to a big firm in downtown of your city. In Philadelphia, big firm lawyers (firms in multiple cities with 200+ attorneys) typically charge clients on an hourly basis. The hourly rate depends on who is doing the work. If you're having a senior attorney (i.e., a partner) work on your application, be expected to be billed at about $600/hr. If you're having a junior attorney (i.e., 1-2 years out of law school) work on your application, be expected to be billed at about $200/hr. Sometimes, you'll have a junior attorney work on your application and then have a senior attorney review it. This may mean that you're being billed for both people's work, so take a look at your invoice carefully. I hear that it's more costly to hire patent attorneys from Washington D.C. and Silicon Valley area.
If you can't afford large firms to represent you, you may want to go with a smaller firm. Smaller firms often offer flat rates, which often makes things easier for clients. But be sure to read the fine lines. Those flat rates may get you only so far. You may be billed for any services that are outside of the original scope of the work. Additionally, remember that you get what you paid for, so don't skimp on a decent lawyer if you can help it.
Wherever you go, you should work with an attorney who understands your invention. This is probably the next thing you should consider, and it may not be a big problem for you if your invention is relatively simple to understand (think consumer goods). You may be more picky, however, if your invention is highly technical or complex. Usually, pharmaceutical clients, tech clients, and other clients who work in hard sciences fall into this category. In my humble opinion, however, I don't think it's essential to work with an attorney having a PhD in a certain subject matter in a majority of cases. Usually, you'll find attorneys with a doctorate or a master's in big firms (e.g., former scientists and engineers). Advanced degrees can come in handy to understand inventions. But remember that all patent attorneys by default have a technical or a scientific background in order to qualify to sit for the patent bar. So the chances are, your patent attorney will probably have some basic knowledge about the field of your invention.
What is more important is to work with an attorney who has exposure to the type of invention that you'd like to protect. When I tell people that I majored in chemistry in college, for instance, many assume that I only work on chemical patents. But the truth is, I've worked on applications for all types of inventions - everything from consumer goods to medical devices, chemical compounds, and electronics. I most often work on software patents and telecommunication patents now, because these are the type of applications I like working on.
The final thing that people consider is location. This isn't a big factor in my opinion, but worth mentioning. Unlike regular state bars, passing the patent bar allows attorneys to practice anywhere in the US. This means that you could be located in one state, and work with a patent attorney in another state. Most of the times, it's easy to work with patent attorneys remotely. I work with many clients from the west coast, for example, and have teleconferences regularly.
Once you have identified someone that you'd like to work with, what should you do? You should probably talk a little bit about your invention and what you're seeking to protect during your initial meeting. Your attorney may ask you some questions about inventorship to make sure that you either qualify as an inventor or an applicant and that you have legitimate interest in the invention in question. I should note that during the intake process, many people are concerned about disclosing their invention and ask for a nondisclosure agreement. While you and your lawyer can certainly sign a nondisclosure, it's probably not necessary as attorney-client privilege applies. After the initial meeting, your patent attorney should hopefully be able to come up with a course of action for you to begin a journey of prosecuting a patent application.