Mindset
A clear mind breeds clear writing. Clear thinking is a conscious act that the writer must force upon himself. A reader has an attention span of 30 seconds, and they are constantly assailed by forces competing for its attention (news, magazines, etc.). It's no use saying the reader is too dumb or lazy to keep up with your train of thought. If the writing can't get their attention, it’s usually the fault of the writer.
The reader who’s confused will usually blame themselves first and go back to reread sentences, but not for long if the writer is giving them too much work. They'll instead wait to read someone who's better at the craft.
A writer must constantly ask, ‘What am I trying to say?’ and ‘Is it clear to someone encountering the subject for the first time?'
Clutter
Avoid the cluttered prose of academese, corporatese, toy manuals, pilots who use phrases like ‘unexpected precipitation’ instead of ‘rain’, university professors telling students ‘we are experiencing explosive expressions of dissatisfaction on issues only partially related’.
From Bill Mauldin's cartoon: 'I'm a simple bum, but now I'm hard-core unemployed'.
Cloudy language is used in politics to deliberately defend opinions that are indefensible. It was during George W Bush's presidency that 'civilian casualties' in Iraq became 'collateral damage'.
By using a pompous phrase, you blunt the painful edge of truth.
Practice
Good writing doesn’t come naturally, you see people saying ‘I’d like to try a little writing sometimes’ or ‘I could write a book about that someday’, doubt it.
Writing is hard work, it’s no accident. A good sentence rarely comes out the first time, or even the third time. Remember in despair, if you find that writing is hard, that's because it is hard
Source:
On Writing Well (Book), chapters 2-3