Washing strawberries is a bit of tricky business. You see strawberries are frighteningly like sponges — they tend to soak up as much water as they can get into contact with. So, long story short, give strawberries a quick rinse in cool water and a swift pat with a clean cloth or paper towels to dry them off when, and only when, you're ready to eat or cook with them.
What's the long story? Well, not to be preachy, but much as they like to soak up water, strawberries are adept at soaking up other things, like pesticides. I, for one, make sure to buy strawberries that haven't been sprayed with toxins. I try to buy strawberries directly from farmers at farmers markets. If you're unable to question the grower directly, you can always look for certified organic strawberries, which won't have been exposed to synthetic pesticides or other noxious stuff.
When, and again, only when, you're ready to eat or otherwise make use of strawberries, put them in a colander and rinse them with cool water. I then like to spread the strawberries in a single layer on a clean kitchen towel or layers of paper towels and pat them dry, as quickly as I can. Then I'll put them on a plate or bowl to eat and serve, or hull, slice, or otherwise serve or use them.
If it's rained recently and the strawberries you've bought have actual dirt on them, go ahead and fill a sink or large bowl with cool water and swish the strawberries around in the bowl until they're clean. Lift the strawberries out of the water (leaving any dirt or grit behind) and lay them in a single layer on a clean kitchen towel or layers of paper towels, pat them dry, and proceed with your strawberry eating or strawberry recipe!