Just as there are things that are considered bad form from artists, there are things to keep in mind when posting a job offer.
Many artists have been burned in the past and are now skeptical about some job offers. If you make your add clear and upfront, you will attract more submissions.
Here are a few pointers:
-If you are expecting to pay in installments, to give stuff (such as copies of books) as part of the payment or have any special form of payment (eg. you only use paypal) in mind when posting the offer, please mention it since not all artists are interested in these kinds of deals.
-If you intend to pay in percentage of profits but your product is not yet sold, this counts as a non-paying job because the artist has no way of knowing they will get paid and how much.
-Indicating the range you are willing to pay, or indicate if you are on tight budget when asking for quotes is considered good form.
-A contest, no matter how cool you think it is, is not a paying job offer. Accordingly, if you decide to post it, put it in the non-paying jobs and indicate clearly that it's a contest.
-Asking artists to do part of your assets as an art test is poorly received by artists just as asking upfront that all submissions contain an art test even before having seen the artist's portfolio.
*edited for typos*