First and foremost, it is important that any risk transferred away from the public sector to the private sector must stay transferred. Much of the ongoing financial meltdown is the direct result of entities who thought they had contracted away financial risk and found it coming back - often at a most inopportune time.
In the PPP context, the state must 1) offer no loan guarantees or similar arrangements; and 2) ensure that each of the private participants has sufficient equity in the project to properly align their incentives and manage the downside risk.
The problems surrounding London's Metronet illustrate what can go wrong when these principles are ignored.