Success should be defined relative to your category. If you're marketing a game your criteria will be different than if you're launching a 99-cent weather app. For example, take a look at download numbers required to enter top 25 in a few different categories:
http://thechocolatelabapps.com/downloads-app-store/

Your strategy to get there will have to also depend on your vertical, but generally it involves a mix of paid user acquisition (via mobile ad networks), organic acquisition through social, viral and web, and finally look at potential for dominating App Store search (ASO).