Kale is a sturdy green that keeps well for several days, even if it's tossed with dressing. Spinach is softer, but holds up reasonably well for a day or two. The best way to store leftovers is in an airtight container for up to two days. If you want to store the salad longer (up to 4 days) then keep the greens and dressing separate, and toss the salad right before eating.