I think it helps a lot to have something interesting to get the attention of any testers. Whether it's a interesting or well made art style, or a gameplay idea that's different than the norm. In any case, you should also put a lot of effort into your post calling for testers. If you don't put in the effort then no one is going to want to put in the effort to test your game (especially if you're trying to get people to test it for free).
You can post here when you have something, reddit might also be a good choice.