There is a lot of good advice above.
I would add that it is important to plan out your marketing. As others have said start looking at how you are going to market the app from very early on in your development. You need to think how your product fills a customers need. That will define how you will market your product, as you can state with asking yourself a simple question of why would they use it then from there move to looking at (if you can afford it) ad space to target which will hit your demographic and target audience.
Hope that helps a little.