The best relationship develops when the buyer acts as a mentor to the new supplier (old suppliers know the ropes and can cope). Some suppliers are proactive in developing systems that please and enhance their supply capability, sometimes the Buyer instigates action and change which should benefit both parties.
There has to be integrity and fairness, where a supplier spends a lot of money catering for a new system the buyer should recognise this with loyalty. Eventually competition may tempt the buyer to offer the contract to other parties to tender, fairness demands that the buyer advises the supplier that change is about to happen so that he can reduce his stock in case he fails to win the new contract.
Lean philosophy should not break a supplier, it should enable him to make a reasonable profit and develop. Loyal suppliers are like gold especially ones that can cope with the supplier pressure.
My tip? look after your suppliers and they will look after you.
The above article is devoted mainly to the subcontract manufacturing part of industry.