Problem: Your loving toddler cannot even bear the thought of a single moment without you, but you've got to run into the store for 30 seconds, and that 30 seconds would stretch into 15 minutes if you brought her instead of leaving her in the car with her father. Or you forgot something in your home and need to slip back inside to get it for a moment, and that moment wouldn't exist should you drag your toddler with you. So, you leave them outside with your trusted friend for just a mere moment. But, no. Tears, screams, where's mommy, the world is ending.
Solution: This worked for me randomly earlier this summer, and I've been using it since. Tell your kid to count you down. If they know their numbers, even if they don't but they know some numbers they can list randomly, this will give them something to do while you're gone. They can concentrate on counting instead of your absence. Toddlers also tend to think they have control in places they do not. They will firmly believe that it is their counting and not the passage of time that brings you back. Once they feel like they are in control of the situation, they feel more secure.
Problem: You have to drop them off at preschool or daycare, and there's no way they can count to 10 or 20 enough times to bring you back.
Solution: Tell them to count you down anyway. At this point, they are used to you reappearing after this game, and so it's not so much the numbers that are important. Count you down becomes toddler code for, I'm coming back. Don't worry, I'm coming back. I've found it works better than me actually telling them "I'm coming back." Maybe it's the shift of control I mentioned above. I'm coming back gives the control to the adult. Count me down gives it to the toddler. Even though they'll not be counting when you go to pick them back up, I find that my toddler remind me they counted me down so I came back. It's cut down on many tears and turbulence.