"From his fullness we have all received grace upon grace." John 1:16 (NRSV)
Robertson's Word Pictures sums up this vision wonderfully:
Grace for grace (charin anti charitos). The point is in anti, a preposition disappearing in the Koine and here only in John. It is in the locative case of anta (end), "at the end," and was used of exchange in sale. See Lu 11:11, anti ichthuos ophin, "a serpent for a fish," Heb 12:2 where "joy" and "cross" are balanced against each other. Here the picture is "grace" taking the place of "grace" like the manna fresh each morning, new grace for the new day and the new service. Emphasis Mine.
Each day we receive a new portion of grace, enough to sustain through the trials and tribulations of the day ahead. The grace of yesterday is gone. The grace for tomorrow has not yet arrived. The grace for today is sufficient.
How often in life to we agonize over yesterday or fret about tomorrow, when God have given us, out of his abundance, more than enough to last through today?