Keeping People Who Visit
First, take several of the best parking spaces and mark them for visitors only. If the parking lot is crowded, attendants could help regulars and visitors to park so as to make effective use of the space and to find empty parking spaces. The welcoming begins in the parking lot.
Second, have parking lot greeters. Preferably a man and a woman who enjoy meeting people. They just walk around the parking lot, greeting all arrivals. They offer a ready and friendly smile. They provide directions for visitors. This is particularly true if the door most people use is not obvious. An unchurched person may not try many doors before leaving.
Don't do things that make visitors stand out in a crowd. Only churched extroverts really like that.
Third, design a strategy to get the names and addresses of first time visitors. An attended guest register may work for you. You need names and addresses to do follow up ministry. We firmly believe a visitor who chooses to attend a church, that is not a church of choice in the community is expecting to have some personal attention.
Fourth, divide your sanctuary into at least four segments. Recruit a person to be responsible to go up and greet every visitor in their area either before or immediately after the service. There should be no nametag indicating those folks have a job. That defeats the purpose. We don't want anyone to visit your church without at least one person greeting them in addition to those who work the doors. This can't be left to chance.