Here is the good news: most app developers spend a lot of time polishing their apps, but very little time marketing them. This is why mobile app marketing represents a big opportunity.

The key to success on the App Store, is to keep your marketing, product management and engineering efforts very tight.

Here are the most important lessons I learned when marketing iOS apps over the last 5 years.

Master App Store SEO or "ASO"
ASO is to the App Store what SEO is for the web: the process of improving the visibility of an app, thus driving organic downloads.
Apple gives you 255 characters to name your app, and 100 characters for your list of keywords. The keywords included in your developer name, app title and list of keywords are used in App Store search results, so pick each one of them wisely. Avoid using special characters in your app title, so it can be picked up by Google and give you some web SEO boost.
The best keywords 1) are relevant to your app 2) generate a large search volume 3) face low competition.

Here is the app download chart of an app I recently ASO-optimized for a client (left half is before ASO work, right side is after):

A/B test your app icon, screenshots and description
Create mobile landing pages showing different assets you want to test, then buy enough traffic to get statistically significant results. The first screenshot on the App Store is the most important one, so be extra cautious when selecting it. A common practice nowadays is to replace "real" screenshots by poster-like screenshots, which may include the main value proposition.

Localize your app, or at least your app metadata
While the U.S. still accounts for 43% of total iOS app downloads, the share of non-U.S. markets is growing fast.
You can limit localization costs by keeping your in-app copy minimal, opting for images over text when possible. Even if your app is not localized, you can still use keywords, app titles and app descriptions for each of your target markets.

Keep your app under 100MB
iOS apps with a size greater than 100MB will have to be downloaded over WiFi.
If you can keep your app size under 100MB, iOS users will be able to download your app over 3G or 4G, which will increase your download conversion.

Implement in-app analytics
Your goal is to find out what your users do within your app. Cohort analysis helps you understand how your users behave, and funnel analysis helps you pinpoint the areas with the biggest room for improvement. Therefore you will make better decisions when allocating engineering resources and will be better positioned to improve your communication around your app moving forward.

Ask your friends to download and review
In order to find out which of your friends use an iPhone or an iPad, type the following query on Facebook:
"My friends who use Facebook for iPhone"
"My friends who use Facebook for iPad"
Since Facebook is the most popular app on iOS, this will give you a close to comprehensive list.

Leverage your current assets (other apps, websites, blogs, email lists, etc.)
If you have an existing app with relevant active users, cross-promote your new app on it.
If you have a website or blog, add a link to your new app.
If you have an email list of relevant users, promote your new app to them.
Keep in mind that mobile-to-mobile tend to convert better than desktop-to-mobile or print-to-mobile. So when sending emails, pick a time when users are more likely to read them on their phone, like before 9am, around lunch time or after work hours.
Instead of using direct links to the App Store, use an affiliate link. Apple currently works with Linkshare in the US. Add "&u1=" at the end of each link, followed by a specific tag, so you can track your source. For example: &u1=From_app1_To_app2 will later inform you how many taps and sales were generated from app1 to app2. Bonus: you will receive a 5% revenue share of all subsequent transactions for any App Store or iTunes purchase for a period of 48 hours or until the user clicks on another Linkshare link.
Update: Apple just announced they are transitioning away from Linkshare to PHG on 10/1/2013. The new program terms include a higher 7% commission on eligible sales within a 24-hour purchase window and the "&u1=" + parameter will be replaced by "&ct=".

Prompt happy users to review
There is the right and the wrong way to do this in my opinion.
The right way is to prompt the user right after they experience something great within your app. For example, they just beat their high score, they just received an answer to a question they asked, etc.
You can also ask users how is their experience with the app has been so far. If positive, ask them to review in the App Store using a direct link. If negative, ask them to send you an email. The idea here is to spread positive experiences and control negative ones on a one-on-one basis.

Read and Act on your Reviews
Some of the most powerful reviews I have seen tend to come from disappointed users who experienced something amazing afterwards. For example, you took the time to address their issue, you pushed an update addressing a popular request, etc. Show your commitment by improving the product and addressing what your users tell you.

Run Mobile Campaigns
Depending on your budget, run campaigns using different ad networks. The key here is to track each of these campaigns, so you can repeat successes and avoid the same mistakes. Also, find out what your users have in common, what your paying users have in common, and keep refining your targeting based on that. Your advertising spend should vary based on what your goals are. If your goal is to learn, don't spend more than a few thousand dollars. Once you figure out what your Life Time Value (LTV) and Cost Per Acquisition (CPA) are, you can adjust how aggressive you want to drive downloads. If you first ASO-optimized your app, your eCPI (effective Cost Per Install) should go down, which will give you more downloads for your money.

Create a promotional video
Videos are the easiest way to communicate the value proposition of an app.
Create a promotional video or app demo video either on your own or using one of the specialized agencies. When sharing your video on YouTube, include the link to download your app on the first line of the description using a tagged affiliate link to track conversions.

Create a web landing page
Such a landing page should be mobile friendly and include smart app banners. App banners make it easy for users to discover and download your app from a website on iOS devices. Add your video to the landing page as well.

Summit your app to targeted bloggers and journalists
An endorsement from a influenced blogger or journalist who is relevant to your target audience can help you boost your app credibility. The additional downloads may be limited, but you can add quotes to the app description and other marketing materials to boost your download conversion.

Check what apps your users have and act accordingly
By inserting a few lines of code, you can check which apps your users have installed on their device. For example, you can check which competing apps they use, offer them a more personalized flow (Ex: no need to show Facebook Connect if the Facebook app is not installed), but also better target them based on who they are. For example, you could figure out how many of your users are single, based on whether their use dating apps or not. Finally, you could close a direct advertising deal with apps that are most popular among your users to target similar users.
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