My pricing for children's parties is based on the number of children attending, the age range and the length of the show. For adult functions, the pricing is based on the type of show - stage, parlor or close-up strolling magic.
Once we agree on price, date and time, I send a letter/email confirming everything we discussed. The other item I ask of the customer is a final count of the number attending the function. I need this info a few days before the function and helps me bring the correct amount of "giveaways", if any.
I am a member of the Society of American Magicians and attend the Magicians Alliance of Eastern States Magic Convention every year. I also subscribe to three monthly magic magazines which keeps me up to date on magic happenings as well as the latest magical effects.