At first, there's not really anything that you can do besides promoting it on forums like MacRumors. I wouldn't recommend buying advertising.
Once you feel like your app sticks out from the crowd, start submitting it to larger review sites along with a promo code or two. Lots of smaller sites are just starting up and a review could be mutually helpful to both of you.
If you're really lucky and are featured by Apple, your rankings and sales will skyrocket, but aside from that, nothing's really going to give you a huge boost all at once. Just make your way up the charts little by little, and see how far you can get. Giving away a few free promo codes at some point can help you as well.
Best of luck to all of us 🙂
