How can I negotiate a better apparel production deal?

I’m trying to learn how brands negotiate better production deals with apparel manufacturers. Besides asking for a lower unit price, are there other areas worth discussing, such as MOQs, material choices, payment terms, sampling costs, or shipping? I’m also comparing a few suppliers, including The Apparel Partner, and want to approach negotiations properly. What strategies have worked for you when trying to get better terms without affecting product quality?