Though I've never needed to if I really wanted a particular artist and they agreed to the final price, but wanted some kind of assurances, I would send them to the DA pages of the artists I have hired as references to prove my reliability in paying, but I would not pay upfront. Because for me paying you half upfront for a $100 commission, that's like 10% of my total budget and I can't afford to lose that to someone that takes the money and does not finish the job. Its not a live and learn issue, its about survival as a business proposition. If they can't trust me, I go find another artist, because others are willing. That's how the world works.