Not a lot to add in general, but for one thing I'll throw out there for what it's worth is that you can't go wrong with Paizo. They take open submissions, seem to be quite reliable with payment (heck, evne pay more than most any other publisher), and have the financial status to not disappear in a month or two.
Most of the publishers I've worked with have been fine, but I also know many of them currently can't afford much of any freelance work unfortunately. I've been lucky enough that only one publisher has stiffed me. The contract was "pay on publication or 90 days after acceptance". The publisher is still around, but the book has been indefinitely "coming soon", and I think they still think it's "pay on pub". Tried contacting them and only got silence.
Since it was only the once, I've just let it go. Also helped that the amount was low enough that the amount of time it would take to keep trying to contact them would be more profitably spent writing or other freelance work (I do some web design). But if it was pretty rampant, I'd certainly start getting more active about contacting them, and failing that stick to more reputable publishers and be clear on terms and contract up front (reputable publishers should have a written signed contract up front - if they don't, I'd worry given your luck).