meatpulp here from my artblog to show i am an artist and know things. anyway. (also. i personally have not been part of a kickstarter, but have been part of some zines and other collaborative creative projects, as well as involved in circles where i have been adjacent to watch the development of these things. i am also just one person.)
It's a tough situation, and greatly depends on the artist you have in mind for the project. (Do you know eachother personally or do you just really like their work? Are they a hobbyist or industry artist? Do you know if they are already busy? etc)
As I am sure you are already aware by the wording in this post, is that any artist would (or should) be wary of big promises in the future return for working for free now.
The first thing I would do, (if you have not done so already,) is create a timeline and budget for the project. You don't have to have it all EXACTLY figured out, but have your baseline ready- how long do you expect each stage of this to go and what are the costs (give yourself some wiggle room for emergencies and delays, people would rather be pleasantly surprised something took less time, than frustrated because something took more time). You are going to want some extra wiggle room around the parts where the artist themself needs to contribute to the project just in case, both in terms of time and money, to allow them to negotiate.
Then, do you have plans for the marketing? Do you have potential manufacturers picked out? etc. Do you plan on hiring on anyone else or is it just you and the artist? Does the artist need to help you with anything more than making the art? Prepare your baseline ideas about compensation and how it will be split, as well as what the potential ownership rights are for the entire project and the art itself-
How is the art involved in this project allowed to be used, before, during, and after the project is complete?
Are there pieces/products that will ONLY be for the kickstarter? (Have you included manufacturers / printers / etc for this part of your planning as well?)
Are there some things that will be available to be reprinted/sold again later?
Who is allowed to do that and what are they allowed to gain from it if anything- and do they have to split their profit in some way?
As well, most importantly: What is the minimum amount of money the kickstarter needs to make in order to actually deliver on its product? (Both without the artist, and your guesstimate of with the artist.)
The more evidence of your planning when you approach them, the better. Try to have a mini portfolio of your project- imagine you are about to present your idea to an executive who doesn't necessarily need (or want) all the details, but needs to see you've got it all figured out (and can answer any questions they might have) before they greenlight your pitch.
Having this stuff and information ready (and available to share) could give the artist more confidence in the potential success of the project, and that it might be worth the risk. Be clear with them about your timeline and expectations. Especially for the initial art.
Do you expect this free-ish art to be done in a few months, or a year? [<- more on that in a little bit]
How much art is needed (min AND max)? Now and later? Do you know the subject matter/sizing/composition/etc?
If/when the kickstarter succeeds, how flexible is your schedule/timeline in comparison to the availability of your potential artist?
For the parts that you are very open to negotiation about- be clear and upfront about it! And same for any spots that you have any uncertainties about- be clear about the reason for the ambiguity (and/or, if relevant, that you can come back with a more certain answer before a decision is made).
ALSO. Do not dump the intricacies of these details on them right out the gate (you just want them ready, if you can), the most important thing to get across in an initial message is your openness to negotiation and eagerness for transparency. And also, including how patient/flexible (or not) you are about the project.
For YOUR protection, I would encourage you to be clear that you would like to set a deadline/agreement of some sort about when the pre-kickstarter art will be completed (as well as what it will be worth retrospectively IF the kickstarter succeeds. This is good for general record keeping, as well as just in case something comes up in the production process and you need to switch artists- that this initial art still gets compensated).
And if that deadline isn't met, no hard feelings on either side, that you will just seek out another artist. This is so you don't get stuck waiting around with someone who wants to participate but doesn't actually have the time to work with you. Which will still mean a delay in your plans, but at least you have accounted for it.
As well, if/when you find an artist, make sure you have it mutually communicated in writing whenever a decision is made or the outcome of an agreement/disagreement- even if you end up getting into voice or video calls discussing things, have some kind of written documentation of your mutual plans. (This can help/protect the both of you in a legal situation, as well as make for better communication and time/record keeping in general.)
for a FULL script of the your entire opening, (potentially, of course, you can edit or disregard this.);
"Hi, [ARTIST] , I am a fan of your work and was wondering if you would you be available in a [TIMELINE] project creating [TYPES of] art for my RPG system and its kickstarter? I have [ALL? MOST? SOME? of] the details figured out with regard to timeline, budget, [OTHER, RELEVANT, THINGS], and what my ideas and needs are for the art. The RPG itself, the layout, and the kickstarter as you can see is [ALREADY? ALMOST?] ready to go- I just need an artist.
I am interested in a generous profit share with you, including [ANY NOTABLE OWNERSHIP/OTHER GAINS/PRIVILEGES FOR WORKING ON THE PROJECT?] as well I am open to further negotiation and discussion about your compensation, licensing rights, and any other questions you may have. I have a great deal of respect for art/artists and want to be clear that you would be joining this project as a partner to its success.
However, and I completely understand if you can't work with me because of this, I currently do not have the funds to dedicate to the initial kickstarter art itself, and the compensation on this project relies on the success of the kickstarter. I completely know and understand that I am asking you for free work now in exchange for big gains later, which is why I want to be clear about my desire to be as transparent as possible as well as open to negotiation about the timelines, [ETC,ETC,ETC] for the project.
Right now, I am only asking for the art to set up the kickstarter itself, which would be [NUMBER AND TYPES OF ART]. Would you be interested in this project? I completely understand if not, given all that I have said. If you have any questions about the project before you make your decision I am open. And regardless, thank you greatly for your time and consideration. "
Then if they aren't interested, you move on. And if they have some questions, you can answer them and/or clarify somethings, add more details as you think is needed, etc.
Depending on the artist you are reaching out to, whether or not they are listing their commissions or availability to work may or may not matter- just shoot your shot and really the worst thing they can say is "sorry I am not available for that right now". (Or possibly purposefully or accidentally ghost you, if you choose someone particularly busy / popular. At which case, give yourself a set amount of time for them to reply, and if they don't get back to you, find someone else.)