When I’m getting ready for an argument with a man I spend one-third of my time thinking about myself and what I’m going to say, and two-thirds thinking about him and what he is going to say.

Abraham Lincoln (1809–1865) American lawyer, politician, US President (1861–65)

Remark (Spurious)
    (Source)

Widely attributed to Lincoln, with no documentary proof or even decent secondary sourcing of his having said it. The above is the oldest version of the saying I can find, an epigraph to an essay, "Tenth Ginger Talk" by Worthington C. Holman, Salesmanship Magazine, Vol. 4, No. 6 (1905-06). From there and to the present day, the quote is a favorite in sales and management literature (it was quite big in the 1920s).

Beyond lack of any near-contemporary citation, a sign of the attribution being spurious is that the saying exists in a wide variety of forms -- "an argument with a man," "fixing to argue with a fellow," "persuade a man" (or "person"), "reason with a man," etc. It's sometimes "When I am getting ready," or "When I get ready." Sometimes the "about myself / him" clauses are left out. Sometimes the order of the division (myself vs the other person) is reversed, as in this variant:

When I get ready to talk to people, I spend two thirds of the time thinking what they want to hear and one third thinking about what I want to say.

All of this points to a fabricated quote with no fixed reference to restrict its form.

Note that the saying is structurally similar to a similarly spurious quote attributed to Lincoln about tree-chopping.

Forms of this are also attributed (with like lack of evidence) to Émile Coué (1857–1926).