Most important tactic: break your product into components and have separate vendors prototype each one. For consumer electronics, for example, you can separate the product into the PCBA, display, plastics, wiring, battery/power supply. Most other products can be broken down the same way.
Try to find suppliers in reasonable proximity so it's easy to drive from one to another. Never let them meet each other. Have your own driver and translator to make sure your IP stays your own.