Well you can use any method that exist. All depends on what product you want to sell. If you want to sell some webmaster product, you better advertise on webmaster blogs, forums,... buy a banner or link on webmaster site. If you will be selling pills you should advertise on health related sites. You can use cpc advertisers like adwords (google), bidvertiser,... or you can buy links or banners on that site. You can post on on that product related blogs, forums,... put link in your signature,... Well this subject is too wide to tell all ways you can promote products.