At the time of this writing, I have two apps on both the App Store and Google Play called "Hyperspace Pinball" and "Tiltz!" They were huge successes, and I'm going to give you advice on how to make awesome games, too!!!!!
...well...actually...they weren't even close to being huge successes, and I don't know how to make your games awesome. Sorry! What I can do; however, is give you a list of things to think about when developing a mobile app based on my own experience. I'll assume you've already heard the parts about your game needing to be fun, original, addictive, targeted toward your audience, awesome graphics, sound and gameplay, etc.:
Goals
If you're making a mobile game just for the fun of it, then do what makes you happy. If you prefer a more business-oriented approach, then study other games, build your game around monetization, and even consider joining an established team to see how things work. Just remember why you got into mobile game development later on!
Competition
When your game goes on the market, you're competing for attention against big companies with major muscle, small developers who achieved major muscle, and every other fish in the big ocean that is the mobile market. Every day, more games enter the market. "Well I just wrote a little game for fun" you say? That's great, but your game is still competing :). Keep these things in mind when you're scratching your head over the lack of downloads.
Monetization
Do you want to make money with your game? If so, you need to make a plan for it early on. Ads earn you money if you have a LOT of people PLAYING your game. Apps that charge for downloads earn you a lot of money if a lot of people see your game, the screenshots are great, the reviews are great, you have a good reputation, and people know they'll enjoy it without exception. In-apps can earn you a lot of money if people thoroughly enjoyed your game, and know they'll get their money's worth for the extra features. The majority of my income came from 99 cent downloads from the App Store and I plan to move on to in-apps for extra game content. I advise against using an in-app purchase to upgrade a free version of a game to a full version. More people will rate you for the limited content of the free version, leading everyone else to think that the full version isn't as good as it is.
Complexity
Are you making a simple word game, a side scroller, or a big 3D production? The more complex your game is, the harder it becomes to maintain on all devices. Case in point: Hyperspace Pinball uses 3D rendering and some shaders. The aliens won't appear on the screen on some Android devices, and other devices stutter or outright freeze or crash during game play. I don't know how to fix everything, and it's resulting in negative comments and a lower rating. A word game, on the other hand, probably won't have those problems if it gets proper testing on at least one smartphone and one tablet.
Choosing your game engine
When looking for game engines to use, look for ones that fit your needs, works fairly well out of the box, and also has a large community backing. If you're inexperienced, consider finding someone to team up with. Please don't spam chat rooms and message boards with simple questions; Google is your friend. Don't let me stop you from writing your own engine, but remember that if you use someone else's, you get the advantage of less work and a lot more people having tested it.
Performance (Speed)
Speed is never a non-issue. Even if your game is a crossword puzzle, it must be totally responsive to the user and appear to run very smoothly. Even if it's fast on your device, test on other devices. Case in point again: Hyperspace Pinball's first rating was 3 stars out of 5 in-part because it was slow...except on my smartphone which ran it just fine. Fortunately I managed to fix the major slowness, though I still get reports of stuttering on occasion. One other thing: try not to let your app take more than five seconds to launch.
Different devices to support
If you're making an iOS game, you only have to worry about a handful of different phone and tablet-sized devices. If you're making an Android game...good luck. What works for you may not work for someone else. If you're inexperienced, don't release your game with major fanfare at first; let it get in the hands of a few hundred users first. Read user comments and bug reports, and make any fixes you need before really going all out on marketing. I've personally worked with two users who reported bugs; and provided workarounds for both.
App Size
I know I'll get instant gratification when I see that an app is only 1-3 mb to download. If it's 50 MB, on the other hand, I expect a full-featured and possibly-clunky game that will deliver weeks of unrecycled play and near-infinite content. Things are fine for me when I stay in the 20 mb range; but I generally try to keep my games as small as possible.
Ratings, initial ratings and feedback
The problem with mediocre apps is this: People aren't excited enough to rate them highly, but those who don't like them or encounter bugs are quick to rate them poorly. The App Store deals with this by hiding ratings until enough come in, and also separate them by version. Google Play just shows all the ratings for all versions. Imagine that your first comment is negative, and you get a 1-star rating with it. Congratulations; now everyone on Google Play sees your terrible 1-star game. Pay attention to feedback, fix bugs; and when you're confident, market your heart out so that you get a rating that the market really thinks you deserved.
Unwelcome features
Hyperspace Pinball got a 1-star rating from someone because they hated the loading screen with ads in it. Another developer told me a story about how integrating a social networking plug-in with bothersome pop-overs caused their downloads to plummet. If you want someone to pay for your game or to Like it on Facebook, be discrete. Don't force them to do it, but make it really easy for them to do.
Development costs
Making an iOS game? Be prepared to buy a Mac, an iPhone or iPad, and a developer license (kids ask your parents). Making an Android game? Be prepared to buy an Android device...and if you're a really serious developer, you'll need to buy 50 different devices because at least one of them will have issues. Save some money for marketing; those agencies can be really expensive unless you have the time to maintain active social networking feeds ("Free" marketing isn't free; it means you're spending more time than money...of course if your game is good enough, your fans do more work for you).
Release Wait Times
I've usually had to wait a week for any iOS app submissions to get approved. On Google Play, it takes a few hours to update. The advantage of the App Store is that Apple will point out any major issues with your game. The advantage of Google Play is finer control over release times and fast patching.
Marketing
Marketing is a very time and focus-demanding task. Your game is really a marketing tool; you earn your money because your game is something that both creates and satisfies a need, and people discover and understand it. App reviewers can help although many of them ask for money; and most of the rest of them will ignore you if your game is mediocre. Having a website with a "Press Kit" page and a YouTube stream with videos do help, but don't expect many visits unless you're well known. Social networking is hard to use as a marketing tool unless you have at least a small team of people who all take their own initiatives because they're excited about your game, and post somewhere at least once daily. GamesPress.com is a great place where you can post free press releases. When you release your game on the App Store, you get some exposure on the New Releases page. For Google Play, you're on your own. Be creative in your Google search words for mobile gaming news; there's all sorts of places you can post it! I tried building up excitement for my game releases starting two weeks in advance; but nobody really seemed to care each time.
Social networking
I put a link to my Facebook and Twitter feeds in my games several months ago, and got maybe two dozen new followers and Likes because I didn't give social networking its due attention. For starters, don't use links; put the Like or Subscribe button IN your app if you can. Just keep in mind that doing anything to confront the player or unexpectedly pull them out of the game, even if it's in the main menu, will only frustrate them. Some people don't mind, but others really do. I've found iOS players to be more tolerant of this than Android users. Look for Facebook pages and groups dedicated to mobile gaming; the more specific it is to your game, the better. Add yourself to lots of Twitter feeds of people who often write about your kind of mobile game, too. After you've done those things, mention your game in those streams. Don't spam them, just look for an occasional opportunity where your input about your game fits into the discussion perfectly.
Developer community
Make a page for your game on IndieDB. Look for chat channels or forums where you can talk with other developers, learn from their experiences, get questions answered and maybe even recruit help. Enter developer mobile game competitions. If you win that's great, and if you lose you still get a lot of exposure. Make a pilgrimage to GDC if even once. Just don't feel compelled to have to read every blog and every article from every developer on every website. It's OK to miss one or two as you spend your time coding.
There you have it. I hope that this has given developers entering the mobile market something to think about.
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