Hi, @steemflow! So far there are 3 ways I know of, and I've tried 2 of 3.

1. You can apply when they open their application.
2. A fellow sndbox member can nominate you.
3. You can join and win contests.

I've tried numbers 1 and 2, and I've been accepted twice, too. Although I wasn't able to grab the opportunity at first cohort. :) If you have more questions, you can always check out @sndbox page since they always put updates about the incubator.