Offering to pay an artist is always good if you're looking for a professional who is looking to make a living on his work. It is also about a good -- meaning healthy -- working relationship. You're going to want to establish something that works between you. Ideally, you'll want to get along well creatively.
Speaking from experience in the past, to me this usually means a writer who isn't going to micromanage every aspect of the art, since the direction of the art should be the domain of the artist. The writer is like the scriptwriter of a film, the artist is its director, to put it in different terms, in a different medium. Most artists work best when they're able to make calls about the visual composition, page layout, and general design.
Be flexible. Be willing to work with someone. Be understanding. Some people work best with strict deadlines and others don't, but you'll probably need some general deadlines and checkpoints, because otherwise unfortunately a lot of people don't get things done.
I'm glad at least that you understand from the get-go that for an artist concerned with his career, it is going to be at least partially about money. We do have to live. But professional is not just acting like nothing excites you or offering people money. Professionalism is more about respect and courtesy than those things. You can be enthusiastic and informal and still be professional. (But as blop noted, it's great to show your enthusiasm through giving them a context and such -- just don't smother them with specific expectations; as I said, the artist should call the visuals.)
Respect your creative partner as you would like to be respected. That's the best advice in one I can give.
