You should first remove any bad leaves, including leaves that are slimy or yellow (via Delish). Those leaves are already starting to rot, and keeping them will cause the rest of your spinach to start rotting too (via The Natural Nurturer). You may or may not want to wash your spinach before you store it. If you do decide to wash it first, make sure your spinach is completely dry before storing it. My Recipes recommends using a salad spinner, warning that it’s the extra moisture that causes spinach to get slimy.
You now have three options for storing the spinach in the fridge: wrap the spinach in paper towels and store it in a plastic bag, line a plastic container with paper towels and store the spinach on top, or place the spinach in a plastic bag and inflate it with a puff of air before sealing. Kitchens Ready found storing spinach in a plastic container with a paper towel to be the best method, helping to preserve the spinach for up to 10 days. Finally, if you still can’t finish all your spinach during that time, you should try blanching and freezing it for later use (via Bon Appetit). Now you have no excuse for not making that green smoothie. Popeye would approve.