I blast out a quick blurb and the link to the product on Google+, specifically hitting the Communities that apply (i.e Dnd Fans, Dungeons & Dragons, Dungeons & Dragons 5th Edition, and about a half dozen others, all easily discoverable on a search).
I also link to it on Twitter, and occasionally chat with or follow RPG-related content creators, publishers, and other bloggers like Sly Flourish and the like. Use #DnD and the like and eventually you'll make some connections to help blast it out for you.
Press Releases on the homepage of ENWorld.
Add it to my signature line on RPGnet (I actually just have a link to myself as an author on DMsGuild, and list the products without links, keeping it shorter and simple).
I don't use Reddit enough, but hoping to change that.
I have my own blog -- which came first and had 40-odd posts before I started doing DMsGuild stuff -- so people already look there. I often post about the product on my blog, and link to DMsGuild within that post...but I don't always send folks to my blog. See, the blog helps me post screenshots in a manner that's better than DMsGuild (sorta), but it also means people have to click twice to get to the product, which is a barrier. It's an art and not a science to know when to do this.
I then use Google Alerts to set up alerts about similar key words and titles. Whenever a pertinent conversation comes up, I take a peak. If it's relevant to drop a link to my product as a comment or whatever, I do so, but only rarely: spamming conversations is mean. You gotta engage first, drop links second (if at all).