There is NO simple answer for this. Is the app a game? Is it a utility? Is it a social app? All apps have different advertising/launch strategies. On a broad level, Facebook's in-stream mobile app ads have proven to be very effective for conversions so I would probably start there. The key is to concentrate your efforts in the first 3-4 days of launch for advertising so you can drive up downloads and app reviews in a short period to (hopefully) be recognized in the "new and notable" and "what's hot" categories in the AppStore. Doing that effectively elongates your launch by getting in front of more people and increasing downloads.