One of the common elements of agile methods is that they raise programming to a central role in software development - one much greater than the software engineering community usually does. Part of this is classifying the code as a major, if not the primary documentation of a software system.
Almost immediately I feel the need to rebut a common misunderstanding. Such a principle is not saying that code is the only documentation. Although I've often heard this said of Extreme Programming - I've never heard the leaders of the Extreme Programming movement say this. Usually there is a need for further documentation to act as a supplement to the code.
The rationale for the code being the primary source of documentation is that it is the only one that is sufficiently detailed and precise to act in that role - a point made so eloquently by Jack Reeves's famous essay "What is Design?"
This principle comes with a important consequence - that it's important that programmers put in the effort to make sure that this code is clear and readable. Saying that code is documentation isn't saying that a particular code base is good documentation. Like any documentation, code can be clear or it can be gibberish. Code is no more inherently clear than any other form of documentation. (And other forms of documentation can be hopelessly unclear too - I've seen plenty of gibberish UML diagrams, to flog a popular horse.)
Certainly it seems that most code bases aren't very good documentation. But just as it's a fallacy to conclude that declaring code to be documentation excludes other forms, it's a fallacy to say that because code is often poor documentation means that it's necessarily poor. It is possible to write clear code, indeed I'm convinced that most code bases can be made much more clear.
I think part of the reason that code is often so hard to read is because people aren't taking it seriously as documentation. If there's no will to make code clear, then there's little chance it will spring into clarity all by itself. So the first step to clear code is to accept that code is documentation, and then put the effort in to make it be clear. I think this comes down to what was taught to most programmers when they began to program. My teachers didn't put much emphasis on making code clear, they didn't seem to value it and certainly didn't talk about how to do it. We as a whole industry need to put much more emphasis on valuing the clarity of code.
The next step is to learn how, and here let me offer you the advice of a best selling technical author - there's nothing like review. I would never think of publishing a book without having many people read it and give me feedback. Similarly there's nothing more important to clear code than getting feedback from others about what is or isn't easy to understand. So take every opportunity to find ways to get other people to read your code. Find out what they find easy to understand, and what things confuse them. (Yes, pair programming is a great way to do this.)
For more concrete advice - well I suggest reading good books on programming style. Code Complete is the first place to look. I'll naturally suggest Refactoring - after all much of refactoring is about making code clearer. After Refactoring, Refactoring to Patterns is an obvious suggestion.
You'll always find people will disagree on various points. Remember that a code base is owned primarily by a team (even if you practice individual code ownership over bits of it). A professional programmer is prepared to bend her personal style to reflect the needs of the team. So even if you like ternary operators don't use them if your team doesn't find them easy to understand. You can program in your own style on your personal projects, but anything you do in a team should follow the needs of that team.