By uncovering mutual interests, you can build a relationship beyond the contract that will produce exceptional results.
Define the goalposts. Be clear on what you want at the start of the project and how you will evaluate your supplier's performance. The more detail you can provide, the more likely the firm is to deliver what you want.
Ask for specifics upfront. Find out who they've worked with in the past. Ask what happened and how they coped with problems. Past performance is a predictor of future delivery.