I generally just rinse my empty tanks (without the carto in it, of course) out under running water. If there is still some residue left I use a Q-tip swab to mechanically clean it, too. You could also wash it with soapy water and rinse it good with water.
You can preserve the old cartomizer by capping it with the clear/white plastic condoms that new carto's often come with and taping the punched/lasered holes. This will keep any unvaped
juice in the cartomizer from drying out.