I've had a LONG run of possible buyers who exhibited every buying signal, knew my rates then dropped out just before the deal closed. Is it something in the air? It's never been like this before.
Offer a small initial project - a proposal, sample work, etc. for a reasonable fee. And then set regular milestones with payment due. You can also ask for a portion up front.
If your rate is something like $50/hour, and you expect to bill approximately 20 hours/week for the next 10 weeks -- for a total of around $10,000, asking for $1000 up front is perfectly reasonable.
A sample of work (for example, some initial concepts for a website redesign) for a couple hours work, billed at $99 paid up front after the initial consultation is a good example.
And then set a weekly (or bi-monthly) payment schedule, with acceptance. That way you are never more than a week (or two) behind -- and you stop work if they don't pay.
There may be questions about acceptance of existing work, but you can log a future work request and continue working and agree to address those issues going forward. It's good to put in a buffer of further support into the contract to cover these cases. So that if you end up with an extra month of work above what is expected, you have it in the contract already.
For my test automation services, I offer a free initial consultation and then a proof of concept smoke test with continuous delivery that will run for 30 days for $199. it provides a lot more than $199 value and since setup is mostly automated, doesn't take me too much effort up front. Once they see that I can deliver, we negotiate the larger scope deliverables. And even if we don't reach an agreement, they walk away with something of value, and I haven't lost too much time invested.