Having a good relationship with supplier is extremely important. I have worked in Supplier Quality for the last 7 years. I think what @ap962 posted is a great start but one of the most important things is making sure that you select a supplier carefully who can meet expectations and also making sure that goals are aligned between the your company and the supplier. Some misteps I have seen are when we have taken a supplier and asked them to make something they don’t usually make. They may be a great supplier, but when given a more complex part or device the supplier’s systems can’t handle it. The other mistep I have seen is when we go into business with a company that has a certain expectation and it is not the same as our expectation. Avoiding those issues early (along with the items listed by ap962) will go a long way in making sure you have a good relationship with the suppliers.