The system I use which proves to be very effective is: When I complete a job often times my client is busy and just gets out their checkbook to pay me as I said I was done with the job.  Before leaving I insist that they inspect my work because; I'm proud of my work, to verify I did everything they asked and it it done to their satisfaction, if they have questions before I leave.  It usually only takes a few minutes and saves a lot of calls 5 minutes down the road because someone has a question.