I use the soaking method. Tonight it took three soaks. Don't those bags claim "triple-rinsed"?! I guess that might be the magic number.
Remove any tough stems, then place the spinach in a large bowl or your sink. Fill the bowl/sink with cool water. Gently agitate the leaves around the bowl with your hands so that any sand and dirt can fall to the bottom. Let soak for at least 5 minutes.
Remove the leaves from the bowl/sink while you refresh the water with clean water. Place the spinach leaves back into the bowl/sink and wash them again. Repeat until the leaves are clear of any sand or dirt.
If you plan to eat the leaves raw, dry them completely.