It happens to everyone—a little overzealous buying leads to containerized plants sitting around waiting their turn to be planted. Sometimes a week goes by, and perhaps we forget to water. Now the potting soil is super dry, so much so that it has started to pull away from the rim of the pot!
Assuming the plant is still alive (often it will be hanging on), here's how to really rewet the soil and irrigate the plant before transplanting it. (Remember, it's always best to transplant a well-watered plant, not a dry one.)
If you simply pour water over the soil, it will probably run down the sides of the pot and right out the bottom. It's easier to submerge the entire pot. Just place it in a bucket or similar container and add water until it rises over the rim of the pot and covers the soil surface. You may need to place some small stones on top of the soil to keep it from bobbing up.
Leave it this way—sometimes it can take several hours for the water to really soak throughout the soil and rootball—then remove the pot from the bucket, let the plant rest and drain some, and transplant into your garden or decorative container.