It depends on the type of product. You have to attract the right type of audience, one that will truly engage with your offer and has the potential to become loyal. This is the most time consuming part but it's worth the effort. Start by creating a sales page with email capture giving away something that is related to your product (tips how to use it best, how to's related to your field, etc- something of real value that relates to your expertise/field/product). Then continue to create valuable content by reaching out to blogs, social media groups (reddit is a good one if you know how to use it), influencers -as mentioned by my colleague in here- and promote both your sales page and product.
Have a smart email campaign to welcome and keep those who sign interested/warm until they decide your offer is a good fit for them.