Get paid in advance. It's an increasingly common practice to get a 30-day advance on services rendered, but that's not enough anymore. Insist on a three-month advance for your services. Think of yourself as a landlord who rents to drunken, party-crazed rock stars. So protect yourself.
Most vendors budget their fees for PR on an annualized basis.
Again, this tactic is only worth it for the high-risk clients who may go belly up before you get all your money. Offer introductory rates for new business. Sounds wrong, maybe, but this may be the time to pick up market share and improve the pedigree of your clients. You steal them and make them realize what they've been missing. Soon, other vendors want to know more about the agency that snared Acme.