Who is your target market? If for example it’s students then you can post a job on websites like WayUp, look for interns to help you spread the word. There is a fee on such websites but you do get interns plus people who are interested in your product to try it in the process. If your target market is actors for example then you can do similar things in casting websites...get the idea?
Thanks, everyone, for the suggestions. I think the "niche" suggestions were the most helpful. I haven't had much feedback from Show HN. Just FYI, the project is a mathematical oriented drawing app for Android. It's here: https://play.google.com/store/apps/details?id=com.doodleback...
My side-projects are mostly mathematical, so I used to post announcements on usenet sci.math.research pointing to the page on my website. Now I don't announce at all, I figure that interested people will probably find them via search and spread the word if the code's any good, ignore it if it's rubbish. I get some of each :-)
unfortunately google is not supportive of newish content these days. if you re interested in some feedback , would be worth to simply post on reddit or some other forum, or to a twitter audience if you have the time
show hn > reddit > alternativeTo imho. indiehackers and other small communities are just too little traffic so i feel like i m spamming them. product hunt is either hit, or miss completely. reddit has /sideprojects and /imadethis , but you could also promote it to niche subreddits related to your project