You can "speed up" the process a bit by tugging hard on each string as you tune it up the first time. Grab the string and pull hard in an upwards direction. "How hard?" I hear you ask. Enough to get the string 2" or so up from the plane of the strings. When you do this, the string will now be way out of tune. Tune it up to pitch, and then pull it again! Three or four times and the amount of "out of tune" from each tugging match will be reduced to "barely out of tune," at which time you're finished.
Doing this will reduce that almost-a-week to two-or-three days of not keeping pitch. Playing hard after each tugging session will help, too, as the strings settle in.
If the strings are consistently going flat, after hard playing and such, check to see if they aren't actually slipping on the pegs. You should have several windings around the pegs to assure yourself of staying in tune. "How many windings?" is the cry. At least two full windings for the wound strings and at least three, though four is better, for the plain nylon ones, is my motto.