what are the best methods to promote online products?

Yup. Said it a thousand times, if you sell a product or service, you want targeted traffic. ie a fighting chance that the odd person may put their hand in their pocket, or pick the phone up

That said, as a directory we are actually the opposite and want every bit of traffic we can, targetted, untargetted, redirected, referral, accidental, and then we apply a magic formula of xyz traffic generates xyz revenue from monetization

But targetted traffic for our actual products

Online marketing is the best way to generate more traffic and sale. You should go for social media marketing and video marketing.

SEO - Search Engine Optimisation
SEM - Search Engine Marketing
SMO - Social Media Optimisation
E-mail marketing

Social Media like that Facebook, Twitter, Google Plus. etc.

Run online ads. Google gives you a guide to go about this. If you can do social media and SEO on your own or hire an agency to do the same. This will definitely generate you leads.

product reviews work best if the product's brand is already popular/out there.

Otherwise, I believe ppc is better, or social media marketing ........

Anyone spoke to you about Product Listing Ads (PLAs)? You'll need a Google merchant account and a basic knowledge of XML, or be good at following instructions. I know of a plumbing company who rely on this method alone.