You probably visit at least a few sites that are relevant to your business on a regular basis, so why not join the conversation? Commenting doesn’t necessarily provide an immediate boost to referral traffic right away, but making a name for yourself by providing insightful, thought-provoking comments on industry blogs and sites is a great way to get your name out there – which can subsequently result in driving more traffic to your own site. Just remember that, as with guest posting, quality and relevance are key – you should be engaging with other people in your niche, not dropping spam links on unrelated websites.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.