Keep your clay wrapped in a moist towel. Make sure the towel is not too wet and not too dry because that will affect the moisture of your clay
Keep the clay in an air tight plastic bag so it does not dry out. Make sure to remove any excess air in the bag.
Once you begin a project, to keep it moist, drape a damp towel over the project to keep it in tact.
Once your project is started, put the covered project in a sealed trash bag so it does not dry out and keeps the moisture.
If by chance your project starts to dry up, simply spray it with a water bottle and cover with a damp towel.
To reconstitute your dryed out clay, put the dry clay in a cup of water but only add enough water to cover the clay. Do not put in too much water.
Let the clay sit in the water over night to make sure it soaks up the water.
Now take your clay from the cup and wedge it to help draw out some of the extra moisture. To wedge the clay simply massage the clay in your hands. (Be careful is may be messy)