Show them who is boss. There's a thin line between a supplier and a friend. Make sure you draw it. That makes it hard for them to resort to emotional blackmail if things go wrong. And don't go soft on them; make sure any mistakes are reflected in the invoice.
Keep them on their toes. Markets change, new suppliers spring up and prices fall. Review your suppliers' performance at least once a year and get quotes from their rivals. Every so often, ask them for a cheeky discount.
Pick their brains. Call them in once a year for an update on industry developments. Ask how these could be used to improve their service and your product, and for any other ideas they may have.
Bulk buy. Speak to colleagues about their suppliers and look into placing joint orders. Think economy of scale. This isn't just a crafty way of offloading your work onto someone else, although that may be a benefit.
Know the whole team. Don't be reliant on just one person at the other end, otherwise you could be left dangling if they go on holiday, take sick leave or move on.
It works both ways. Be generous. If they do a good job, encourage them. Write testimonials, forward their details to colleagues. As ye sow ...