Hey you need to be ready to deploy a full arsenal in marketing an application in Apple Itunes. Your not just marketing a downloadable product your marketing something that will be of value for consumers I'm assuming. Whether it's free or paid or even having in-app purchases. Consider Facebook, Instagram, YouTube and Twitter. Be ready to put in hours to make sure you get this app attention.
The greatest ways are Viral facebook apps that revolve around the application itself that will interest the traffic. Instagram is a great way to get followers and traffic to the apps page, if you know how to do it. Make them a quick website (one page) that displays their app and then leads to the actual download page. Get a video done for the application as well (good-high quality is best) and upload that to YouTube and buy a few views on Fiver to get that going. Set up the YouTube page nice don't just slap on a random image as background and leave as, tailor it to the application and edit the about pages and insert the link. Twitter is the same and that's self explanatory. Try the website buysellads, they can allow you to target specific niches and even the top app reviewers in the industry, that's golden. Also bring to mind Media buys. Of course not just any video will do but any video that has a nice amount of views and coincides with the app you want to drive downloads to. I'm assuming you have a budget of at least $100 for starts after all, you will make more money from doing this. I think of it as a guaranteed investment and I will see a ROI. Reviews are a necessity and will result in more downloads in the actual app store. Pay people on review sites a few bucks to leave a positive review on the app after downloading (it will cost them) but you are paying them anyway. Also keep in mind the most important thing before starting is doing keyword research on the actual app store. There are tons of websites that will help you on this and it is definitely worth looking into. Make sure the apps icon image in the marketplace is not some cheapo icon, this can drive away potential clicks and is a no no. Make sure the app has very detailed information on what the consumer is downloading because essentially the more the merrier.
I hope this information was semi useful to you I just wanted to help you out a bit. I do this myself and I have my own twists of doing things which you should learn as well. Find what works, guard it, sap it, repeat it.
Regards,
Saul