At our school we will only accept £1000 up front for SEP type work and £2000 for ME (limiting to £1000 is a bit tight when you could blow that amount in a day). We simply do not let people pay huge amounts up front. Funnily enough, it doesn't appear to adversely affect our business.
Frankly, bonds, escrew etc is still not the long-term solution. PAYG (or thereabouts) is the only sensible way to do business.