You need to consider good, old fashion SEO. Do some basic keyword research to understand how people might be searching for your app - or what it offers - and adjust or develop web content targeted for that search. If you're able to get rankings for these searches, you'll begin to see the increases and I would even recommend setting up a direct link to the store with analytics tracking to see how many leads that you're sending.

Also, it's not free, but advertising is something you should also consider. Apple launched a simple platform - http://advertising.apple.com/tools/iad-workbench/ - a few months back and there are several other app networks that you can use.

Best,

Clark