Read the works of clear thinkers (subject is not so important.)
Two that leap to mind: Wendell Berry and Norbert Wiener
Also, having to refine and articulate your thoughts
Thank you
Lots of LSD
Clarity of thought isn't something I'd associate with lots of LSD. Typically I find chronic users struggle to articulate their thoughts and appear 'spacy'.
Philosophy and meditation.
(imo) Putting things into perspective (e.g. "pale blue dot" )