If your website receives a good amount of traffic, you can seek for advertisers who are willing to pay you for writing a promotional article and advertise their products or services on your site.
Beside direct payment, you can accept free products which you will review. For instance, you are running a tech related blog; you can ask a company to send you a free smart phone to review. This is a win-win situation: you got a smart phone for free and they got a review and will constantly receive traffic for your website.
Once your site has an established group of readers, you have to be gentle and do not annoy them with constant product reviews and promotions. Also try to keep the sponsored content relevant to your website niche.
