I haven't done much in regards of marketing. I tried one of these services that post your app on multiple websites and try to get press coverage but that did not work at all.
Due to the lack of competition the app ranks well on Google for "foosball tracking" and so every week a few people sign up.
Since you can use all important features for free for up to 6 players, there was no need for many to start paying but in February a team finally got enough players that wanted to be tracked and they signed up to the small paid tier. All of you founders here know how great it feels to have the monetary confirmation that your idea is working. The app makes barely any money but over 4,000 foosball matches were tracked already and since we use it everyday in our company I feel it was already a good investment of my time.