I'm a relatively new commissioner to this site, but not a new commissioner. Normally i required half when i start, half when i finish. If I have a backup and can't get to something right away, i only require a small deposit (usually 30-50$) to lock your place in line and show that you are serious, then the rest of what would be half when i get to your place in line. People not being serious is becoming a big problem, especially off of this site. I had a table at a con a year or so ago for props, armor and sculptures, I walked away with 30 potential commissions, ranging from 80$ to 7,500$, didn't even think of doing small deposits, and in the end, not a single one worked out. A small deposit shows your seriousness, and proves that you aren't potentially wasting an artists time. If they end up backing out of the deal, they are only out the small deposit. My next table in dec, will be small deposits required.
One thing ive noticed, specifially here is commissionee's budgets for certain things. I think i saw someone a few days ago looking for a full suit of spartan armor for a budget of 250$. I bring this up because the fact that peoples budgets tend to be so low, someone has to be accepting it. This drives down the price of the services as a whole, meaning more commissioners are spreading themselves quite thin. While this is a problem, this is absolutely not excuse for a lack of professionalism. Replies to correspondence is required for any type of relationship, especially in business and services.
As for the progress pictures portion of this, it's easy for me, because of what i specialize in. Every step is significantly different than the previous. Sculpt - send picture, mold - send picture, cast - send picture, paint - send final pictures and request final payment. I don't do cloth related commissions so i shall claim ignorance as to the steps, but more progress pictures should be sent than just "hey look its finished, i hope its what you wanted"
Etsy store -www.etsy.com/shop/violentevolutionfx