Hey all - I typed this out to help me understand, well... how to understand things:
Mental clarity is the ability to read reality accurately.
I don't mean being able to look at the complete objective picture of an event, as you don't have any direct access to that. I'm talking about the ability to read the data presented by your subjective experience: thoughs, sights, sounds, etc. Once you get a clear picture of what that data is, you can then go on and use it to build or falsify your ideas about the world.
This post will focus on the "getting a clear picture" part.
I use the word "read" because it's no different than reading from a book, or from these words. When you read a book, you are actually curious as to what the words are saying. You wouldn't read anything into it that's not there, which would be counterproductive to your understanding.
You just look at the words plainly, and through this your mind automatically recognizes and presents the patterns: the meaning of the sentences, their relation to the topic, the visual imagery associated with them, all of that. If you want to know a truth about reality, just look at it and read what's there.
Want to know what the weather's like? Look outside - read what's going on.
Want to know if the Earth revolves around the Sun, or vice versa? Look at the movement of the planets, read what they're doing, see which theory fits better.
Want to check if your beliefs about the world are correct? Take one, read the reality that the belief tries to correspond to, and see how well they compare.
This is the root of all science and all epiphanies.
But if it's so simple and obvious, why am I talking about it?
It's not something that we as a species often do. For trivial matters, sure, for science too, but not for our strongly-held opinions. Not for the beliefs and positions that shape our self-image, make us feel good/comfortable, or get us approval. Not for our political opinions, religious ideas, moral judgements, and little white lies.
If you were utterly convinced that your wife was faithful, moreso, if you liked to think of her in that way, and your friend came along and said she was cheating on you, you'd be reluctant to read reality and check if that's true. Doing this would challenge your comfort and throw you into an unknown world with some potentially massive changes. It would be much more comforting to rationalize why she still might be faithful, than to take one easy look at the true information. It would also more damaging.
Delusion is reading into reality things which aren't there. Telling yourself that everything's fine when it obviously isn't, for example. It's the equivalent of looking at a book about vampires and jumping to the conclusion that it's about wizards.
Sounds insane. You do it all the time. You'll catch yourself if you're willing to read the book of your own thoughts: flowing through your head, in plain view, is a whole mess of opinions and ideas of people, places, and positions you've never even encountered. Crikey!
That mess is incredibly dangerous to have. Being a host to unchecked or false beliefs about the world is like having a faulty map of a terrain: you're bound to get lost or fall off a cliff. Reading the terrain and re-drawing the map accordingly is the only way to accurately know where you're going. Having an accurate map is the only way to achieve your goals.
So you want to develop mental clarity? Be less confused, or more successful? Have a better understanding of the world, the structure of reality, or the accuracy of your ideas?
Just practice the accurate reading of what's going on. Surrender the content of your beliefs to the data gathered by your reading of reality. It's that simple.
It can also be scary, especially when it comes to challenging your "personal" beliefs. It's well worth the fear, however, as a life built on truth won't crumble like one built on fiction.
Truth doesn't crumble.