The Monetizing 'Droids series of posts are ways of thinking about the identifying, pricing, marketing, and selling Android applications, blending what has worked in other areas with the subtle nuances that Android imposes.
As developers are quickly realizing, there is no free lunch in the Android Market - applications do not necessarily "sell themselves". For that matter, there is no free lunch in the iPhone App Store, such as this well-regarded game averaging ~5 sales per day at $3.50 gross profit per sale.
Obviously, the nirvana for any Android application developer is to create a "staple" application, one that becomes widely-known and highly-regarded, such that the general advice to Android device owners is to just go buy it. Android will be on millions of devices, and if a nice percentage of those buy applications considered to be staples, those application authors will do astonishingly well.
The catch is, staples are limited not by device memory, but by human memory. Only so many applications will reach sufficient "mouthshare" that they will become staples via word of mouth. A half-dozen applications might be on the tips of everyone's tongues, and a dozen or two more might be mentioned by a decent number of people, but that is it.
Also, staples need to be something that just about any Android owner can use. An improved home screen is one such example. Applications that are relevant only to a niche will not become staples, except within that niche, meaning their success is limited by the niche size more so than overall Android sales figures.
The opposite of trying to build a "staple" is an Orion strategy.
Project Orion attempted to build a spacecraft powered by nuclear bombs. Drop a bomb out the back of the craft, blow up the bomb, ride the shockwave as far as it will go, then repeat with another bomb. You can travel as far as you want given a sufficient number of bombs.
An Orion strategy in software sales is to focus less on single-title sales and more on having many titles. It relies on being able to cross-market your titles, figuring that people who bought your earlier applications may be more likely than the average person to buy your later applications. The goal is to build a following - what Kevin Kelly referred to as 1000 True Fans - the people who will buy each new application you create because they can trust the quality, know it will integrate with their past purchases to add new functionality, etc.
Android's application integration capabilities will help here. OpenIntents.org is a master of this, allowing one application to snap into the next to create a whole greater than the sum of the parts. So, if you are creating a series of games, have them tie into a common "high score" engine (perhaps via an Android content provider and activity to view the scores). Or, if you are creating a series of productivity applications, have them tie into a common "super-clipboard", allowing cutting and pasting beyond ordinary text (e.g., hyperlinks, references to each others content).
Everyone wants the "killer app" or at least the "staple". If you are lucky enough to create one, congratulations! For everyone else, if you are serious about making it in Android application sales, you need to be thinking of product lines and integration and means of cross-marketing, so your business can ride the shockwave of each application release and get further and further along the road to success.