As the commissioner, I prefer the half-and-half method, but as the artist, I only accept payment up front. I had agreed to accept half up-front, and then show them a heavily watermarked version when I completed it for the rest of the money, but apparently the watermarked version was good enough for them, because they skipped out with the rest of the money.
I only do small commissions (four hours work at most), though, so asking for money up front isn't that big a deal.
Depends on how much the commission is costing. If it's a cheap-o sketch commission that's like $20-$30, I just ask the buyer to pay everything up front. If it's a more expensive like say, a full digital painting that can cost up to $300, I always ask for a deposit. Then, as things progress, I usually ask for more money. (aka cash at thumbnail stage, cash at rough stage, cash at rough color, cash at final).
I've only commissioned something once and paid in full up front. I'm comfortable with that as I trust the artist.
I've done some commission work myself, but have always asked for payment upon completion of the work. I'm very cautious of the commissions I take on, which helps with the risk associated of doing it this way. If I were to take on work now, I would do half up front and half at completion.
If I'm commissioning someone, I'll do half and half if they don't have feedback. Generally I wouldn't commission a person without feedback anyway though...
If someone is commissioning me, I'll take the whole thing upfront if they're comfortable with that (actually, I think every pic I've ever done for someone has been paid upfront...), but I don't mind doing half and half.
I think it works to ask for half/half when you're starting, i allways ask for payment upfront but i have made myself a name and a reputation over this past 5 or 6 years, most of my clients know already how i work and many of them are returning costumers and they seem to be ok with me asking for payment upfront because they know im honest and that they'll get the best of me, is not like im goin to dissapear with their money and that's a HUGE advantage for them, but its totally fair for a client to ask if its possible to pay half and half, i do that once or twice a year, when my client seems confused or scared, i try to end up being a good experience by giving up on that.
I have never had a problem when i ask for payment upfront and i take that as a compliment and a reward for all the years i worked impecable n_n
My only advice is NEVER to take work without payment, i did that years ago and i got fooled many many times.... learned the hard way.
it is the worst situation i bet, but i have never been trough it, at the most i get asked to change eye color (slight variations and such) or little things or details, nothing really important, but other than that my clients end up satisfied, im very good at following instructions and i like to respect the character, i think thats key to avoid gettin the work rejected.