How should I keep my work damp in-between sessions?

It is possible to keep clay in a workable condition almost indefinitely as long as it is kept moist.  This is usually achieved by wrapping in plastic. Re-used plastic bags are good for smaller items.  If the piece is left for long periods, there is a danger of it drying out, so periodically unwrap and spray with water. After spraying wait for a minute or so and check the surface of the pot - you may need several applications of water.  Wrapping in plastic is a useful procedure when waiting for pieces to dry to leather hard for turning as it will ensure that the moisture content is even throughout the pot and it will avoid, for example, hard, dry rims and soft feet on bowls. A greenhouse 'tent' (available at garden centres) can be used as a damp cabinet. All gaps should be sealed for best results. Lightly spray the inside walls of the tent periodically to maintain moisture levels.