First, let me state that this is not a definitive guide on hiring all cartographers. Everyone is different, everyone has different needs/wants when working on projects. These simple rules aren't limited to cartographers either as they can easily apply to any artist.
1. Understand Style
To get the best results out of a project, hire an artist who already creates in the style you like. Yes, some artists can create things in other styles but that's not true of all artists. Plus, if their work already displays that style, you know they can easily give you what you're looking for.
2. Communication is Key
As stated before, every artist has different needs of what they need/want in a project. Some need a lot of direction, some need only a vague idea. Some want all the details at once, some only want it in bits and pieces necessary for that step of the project.
Don't be afraid to say, "Tell me what you need from me and how to best give you that information" and then follow those directions.
3. Drafts & Changes
When you hire any artist to create something for you, usually they will send you draft of the art for you to request any changes from. The same is true with cartography. I often give my clients a really rough draft to agree on the size, distance, and spacing of things. I will also just add quick icons for swamps, mountains, water, farms, etc. It's just to give a general idea of where everything goes.
It's during this point that any changes you want should be voiced. Don't be afraid to voice them either because this is what's expected. Artists know that a client will change things once they see it roughly drawn out so that's the purpose of sending drafts.
However, once you approve a draft, requesting changes can cause the artist a lot of extra work and frustration. Its understandable if it's one or two things, but once you start going beyond that, don't be surprised if the artist states they will be adding more money to the price of the piece for redraws.
4. Lean on Their Expertise
The point of hiring an artist is they are capable of doing something you cannot, which is why when they suggest something, you should really take it into consideration. I'm not saying that you should just let them do whatever they want even if you don't particular want it, but if it's something you aren't sure about or have no opinion on, let them do what they do best.
If an artist recommends a change in how something looks, it's usually because they have learned from previous experience that it won't look right or that it will take away from the overall look of a piece. If it's something you feel strongly about keeping, express that and ask if they can suggest something to help keep that feature without making it stand out or whatever the artist thinks will be wrong with the overall look.
5. Respect Their Time
This one is a BIG one that many clients seem to struggle with. As artists are more and more easily accessible though social networks and email, it's really easy for clients to forget that every message they send an artist eats up their time. Even if it's just a quick "looks great" sent via private message/email. If you don't *have* to send them a private message/email, don't. Comments on WIP posts are more than enough to let us know we're doing well.
Also keep in mind that the more popular an artist is, the more of their time is spent dealing with private messages, emails, and posts of people asking them about commissions, their work, prices, etc. This means that if you're trying to hire an artist and they don't respond, it's not personal.
I have over 18k followers on Twitter alone as of this writing. I get HUNDREDS of notifications a day (some times on single posts). I am sent a LOT of private messages from people asking about commissions in one form or another and honestly, I don't have time to sit there and write "Sorry, my commissions are closed" to everyone when it's already clearly on my profile description. So if you don't get the response you were expecting or in a tone that says they're tired of answering these questions, please know it's not you. It's the accumulation of all the messages we get on any given day when we could be using that time to focus on creating our work. Sometimes...it's exhausting.
6. Just the Facts
I tell my patrons to give me "only what's absolutely necessary" for their maps. Give me only the details you *need* on the map. I don't want the area's history or lore. Not because I'm not interested but because I don't want the influence of it if it doesn't change how an area will look. The world wasn't formed around the history of it's people. A map isn't going to show the lich king's effect on the world unless he burned down an area or we can see the ruins of his temp on the map.
Telling an artist information beyond what's needed to make the project only eats up even more of their time and energy. HOWEVER, if you aren't sure if it matters, just ask! Plus, if the artist has time, it makes for great reading and sometimes gives them a much better feel for the look of the map.