Judge everyone in a favorable light. 
There’s a teaching on this “in the name of the Besht.”
I begin with a reservation. I found the teaching in an anthology entitled “The Baal Shem Tov on Pirkei Avot.”  The compiler, Rabbi Y.A. Dvorkes, cites the teaching as having come from a book entitled “P’ri Chayim.” However, there are several books with this title. He (or his translator) gives no further information regarding the author or publication/copyright date.
The author might be Rav Avraham Chayim of Zlotchov. An online mention of his yahrtzeit  also dates his “P’ri Chayim” in 1816. This is after the Besht’s passing in 1760, therefore a possible source. Also, that it’s taught “in the name of the Besht” means that the author did not claim to have heard it personally from the Besht, but received it “orally,” which could also befit a book written 56 years after the Besht’s passing, by an author who wasn’t in the first generation of Hasidim.
As this isn’t known to me as a primary Hasidic source, I can’t be entirely certain whether this is actually a teaching of the Besht’s or not. I assume that Rabbi Dvorkes wouldn’t have included a teaching known to him as spurious, nor would Feldheim be likely to publish a book with questionable sources. So, I accept the teaching as in keeping with much of what the Besht taught elsewhere, even without being fully certain that he gave this teaching specifically.
So, what’s the teaching?
It was taught in the Besht’s name that one is never sentenced — or “judged” — in the heavenly realm to any punishment. “It happens only as in the incident with Natan the prophet, when he came to King David and gave him a parable of a poor man who had nothing but one little ewe lamb …”  He asked David for a judgment (in keeping with his role as king), which David then gave. Natan then told him that the judgment he pronounced on the man in the parable he has actually pronounced on himself for his own previous action!
The Besht teaches that “… a man is shown by Heaven the same [sin] in a different form, as something done by another person … whatever he says in judgment of his fellowman, he is sentenced by that verdict … because unwittingly, he has thus passed judgment on himself.”  “… as you pass judgment, so will it happen to you. Consequently, you should always see to it that you judge [another] favorably, leniently; and in this way you will also judge yourself favorably and leniently.” 
Whether or not this is an actual teaching of the Besht, it’s in keeping with the Talmud:
“All who act mercifully (i.e. forgivingly) toward their fellow creatures will be treated mercifully by Heaven; all who don’t act mercifully toward their fellow creatures won’t be treated mercifully by Heaven” 
It’s the Jewish “karmic” principle of “midah k’neged midah” — “measure for measure.”
The rabbis of the Talmud — later the Besht, too — are showing us how to best use Elul, the month preceding Rosh Ha-Shanah and Yom Kippur, on which days tradition tells us that we’re judged, to prepare for them properly. It’s not only by asking others for their forgiveness. It’s about offering our own — even when unasked.
Our “forgiving” can’t always be simple and instantaneous. More often, it requires considerable effort even to decide to forgive — before beginning the process at all.
We should be willing to forgive, even out of self-interest!
We should also keep in mind a deeper implication of the Besht’s: the Divine, beyond eternally creating us and everything else moment to moment, is also always, at every moment, responding in kind spontaneously to all of our thoughts, words and actions.
Therefore, we shouldn’t fool ourselves by making a show of forgiving while retaining resentment in our hearts silently. Even worse is denying it to ourselves altogether.
Our forgiving should be sincere and thorough.
What if we “can’t?” Or, what if we can begin the process but not complete it in time for the High Holidays?
The Besht is telling us that even our sincere desire to forgive receives a commensurate Divine response.
G-d will even help us to do it, step by step, day by day.
 Pirkei Avot 1:6
 Dvorkes, Rabbi Y.A.; The Baal Shem Tov on Pirkei Avot; Feldheim, © 1974
 II Samuel 12:1-4
 Dvorkes; p. 49-50
 ibid. p. 50
 Shab. 151b ; RH 17a; Meg. 28a