Start now! Just have them spend first an hour, then two and work your way up to the whole night - buy ear plugs if you have to! YOU need to be strong and consistent. Make the crate a fun place to be, toss a toy or treat in to encourage them to go in ... the whole positive reinforcement thing. And whatever you do, NEVER let them out when they are fussing or whining, wait until they are calm, or you'll create monsters!