Aside from identifying your audience, determining what distinguishes the side project from competing or similar apps, turning those distinctions into decent sounding value propositions & getting those out in front of your potential users... getting a spot on HN's front page is always a good start :-) (but it helps if you have all the other work done first so the visitors understand what the app does when you get the resulting traffic)
You could also try and find people whose input/feedback would be helpful to the project & beg them to try the app...this is usually only effective if you've earned their trust by engaging with them for a while...so it helps to be active with your potential customers ahead of time)