While this is just a tightly placed list of what all you can do to promote your app on a budget, there are a number of other things from placing your name for an award to getting the journos and influencers write or review your app, if you are ready to spend some moolah. So, set your budget a little higher and a whole new world will open for you. But meanwhile, start here.
Now that you know what you have to do, do you know when to start?
Well, there is no fixed answer to this. Ideally, promotion should start the moment the idea is conceived, but in your case, when you are already lagging behind, start putting in those extra efforts from the Day 1 of realization.