The prohibition of preparing on Shabbat for the weekday is limited to activities that require effort. However, easy, effortless activities that people routinely undertake are permitted. This is true even if the activities are useful for the weekday, as doing them does not belittle Shabbat. For example, after shaking the lulav on the first day of Sukkot, one may put it back in water, even though this is done to keep it fresh for the next day (Sukka 42a; SA 654:1). One who is studying Torah may insert a bookmark, even though this will not be helpful until he continues his studies during the week. One who took a siddur to the synagogue may bring it home (where there is an eruv), even if he will not be using it again on that Shabbat.
Similarly, one may put leftover food in the refrigerator as usual. Dishes may be left soaking in water as usual to prevent food remnants getting stuck to them. One leaving the house on Shabbat afternoon may take a key and sweater with him for use after Shabbat, though he should not state explicitly that he is doing so for after Shabbat (see SA 416:2; SSK 28:89).
In a time of need, in order to avoid serious inconvenience, one may do simple things on Shabbat in preparation for the weekday even if they are not part of the normal routine, but only on condition that it is not obvious that he is doing them for the weekday, so as not to belittle Shabbat. For example, if one is going to a place where it is difficult to find wine for havdala, he may bring wine with him on Shabbat, on condition that he brings it while there is still plenty of daylight left, so that it will not be obvious that he is bringing the wine for after Shabbat. If he drinks some of the wine at se’uda shlishit, he may even bring wine le-khatĥila (see Ĥayei Adam 153:6; MB 667:5).
In a time of need, in order to prevent a loss, the Sages permitted doing things on Shabbat even when it is obvious that they are for the weekday. Examples include bringing in items from outside and that would likely be damaged by rain, and putting food into the freezer if it would likely spoil if left out (SA 308:4; MB 321:21).