Smoothies always taste the best when made and served immediately. But if you have to then make it a day before and store it in the fridge. But do not keep it for more than a day. To prevent the smoothie from turning brown (due to the avocado), add some lemon juice before refrigerating it. Stir before drinking it. You can also re-blend it in the blender. Smoothies can also be frozen. Transfer them to ziplock bags or Popsicle molds and freeze them for up to 3 months.