Keeping retention

Hey everyone, I'm hoping this thread can help not only me, but the community as a whole.
I am curious if you'd be willing to share, if you continue to read on, you'll get the gist of what I'm asking.

These do not have to be answered, they are just general questions.
1 - How do you go about retaining your player base? I've noticed a lot of games, in general, for example on the App Store, get downloaded, tried, deleted. So what techniques have you done to try to retain them?

2 - Is there a specific technology you used, to make sure its integrated in a way that helps retain users? E.G. - Ad Rewards, perhaps server-side random loot prizes every so often, etc. (or some various 3rd party tech).

This isn't targeted at Mobile games, this is just general examples I could think of, I'm not making a mobile game, but others are, and Mobile tends to be the one that gets installed and deleted fairly quickly.

The two questions above, aren't necessarily the things you have to answer, but just in general, how do you go about trying to retain users aside from just 'making a fun game'. Games can be fun, but not keep players, based on various reasons that may or may not be obvious.