I hate when artists ask for half up front, and it might even scare me away from some of them. I've heard way more horror stories of clients getting ripped off by artists than the other way around, which makes sense: if the artist gets the money up front, they have all of the leverage at that point. Unless you give all the art to a client instead of just a watermarked preview or whatever, they have absolutely no reason not to pay you, except for if they suddenly go broke or if your work ends up not meeting their expectations at all.

I dunno. I bristle whenever I'm asked for money up front. If I'm commissioning work, it's because I want it. There's no logical reason for me not to pay for it a week or two later when it's delivered.

Sorry you got scammed, though. I wouldn't deliver finished, unwatermarked art to clients without getting paid first. I think it's a good idea to do work in small batches too, like you mentioned. This kind of stuff is why I'd never do commission work, and barely ever commission anything. Tough situations. Good luck to you!