For the best flavor, I recommend making your stir fry just before you're ready to eat. However, you can do all of the prep up to a day ahead of time. Assemble the sauce, slice the beef and cut the veggies into bite-sized pieces. Put each component (1. beef, 2.sauce, 3. ginger/garlic 4. veggies) into individual storage containers and refrigerate until you're ready to cook.