One should try one's / his hardest to write the message as clearly as possible.
"One should try one's hardest ..." is BrE, whereas "One should try his hardest ..." is AmE.
Am I correct?
Thank you very much.
All the best
I can't speak for AmE; but both versions are possible in BrE.
However, this use of "one" is felt to be affected, by many BrE speakers, and so is not very common in ordinary usage (except in jocular contexts, e.g. mimicry of members of the Royal Family).
(Some would also object to the use of "his" as the common possessive.)
I think that you should not follow "one", in the sense of "a person", with "his" or "her" only in formal writing (American English). I agree with MrPendantic; using "one" that way does seem affected. Maybe you can replace "one" with "you"? :)