Anthony's point about building a working relationship is spot on, working together to grow both your business's is vital and a good supplier will always want to do this. Once you have established the relationship then you will find that they tend to offer you different prices or 'extras' as they know you are a known entity.
My supplier recently rang me up and offerred me 100 motorola t192's for £500 which we gladly took and are using them as a promotional tool, ie. giving them away to charities etc in return for a bit of free advertising.
Focus on developing this relationship as in the long term it will benefit you, we regularly go to lunch with suppliers and customers alike (in particular B2B customers) as it builds the 'relationship' between the two entities.