What is the best why to get your nipples hard?

There is no scientific evidence to support any method of nipple stimulation as being "best". Nipple stimulation is a highly individual experience, and what works for one person may not work for another. Some people find that physical stimulation, such as pinching or rubbing, is effective in stimulating their nipples, while others prefer the use of ice or heat. Still others find that psychological stimulation, such as thinking about sexual thoughts or fantasizing, is more effective in producing nipple hardness. Ultimately, the best way to find out what works for you is to experiment with different methods and find what gives you the most pleasure.