I had such an experience last two years and it wasn't funny. Since then, I can't work for you without you paying 70% of the work or I won't do the work for you.
We all want the same thing even though there are people that don't keep to their own bargain after paying them. And some clients too are afraid of such to happen to them. I know it's hard to trust who you don't know, benefit of doubts is just a trial.Personally, I like starting the work once you've made payment. I mean it is now my responsibility to keep to my side of the bargain at the end of the day.