It might sound silly, but I want to give credit where it is due for the other cats (and dog) that help raise our kittens.  Our kittens grow up knowing that every person and animal that they meet is a potential friend!  Having other cats in the house helps them learn "cat manners" which come in handy if they are going to a new home with other cats.  Our dog is very friendly and gentle with the cats but is huge compared to them and he barks a lot.  So our kittens are accustomed to his presence and noise!  They still may need time to get used to a new person or animal in their environment, but they will be used to having other animals around.