Well, direct sales is a bit difficult because my core target is "Agencies that use Slack, of which clients use Slack". So I need to convince a layer of users (let say 10% conversion) and they have to convince another layer (10% again) => 1% conversion rate.
What worked was publishing some articles on Medium and optimizing a bit for SEO, so that the handful of people who actually has the problem, can find me. The Slack app Directory helped quite a bit at first, but I should take better advantage of it now that they have add features.
But really, the main growth factor is : almost no churn. People are super satisfied. So even if I don't spend much time finding customers, the very low growth keeps adding up...