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.
Share the success. If the work they have done with you has led to greater things, be sure to thank them and show how their contribution helped steal the show.