Testimonials are great. After your first few sales call up the customers and ask them for a testimonial. You then print this or display this on your website or packaging.

I use social proof in my consulting business. I display my distribution partners and clients using their logos on my advisory and consulting sales page.

In the right column of my blog I place my client testimonials.

You can also create social proof by displaying how many people are customers or how many people bought the product or service. You could add a counter then change the counter every time someone bought the product or service.