It doesn't even have to be tasteful, just a generic feel-good story, some giveaway, or if you're already famous just post on /r/IamA.

> /r/Subreddit helped me achieve my life long dream of [...].

> You guys helped me in my though times and I am giving away [...] in return.

> My name is [Famous Celebrity] and you should buy my new [...]. Btw, I might answer some generic questions.

Alternatively, you can buy upvotes, comments and compromised years old accounts to promote your product.