I used to get guys to reconfirm 2 hours before but I have now changed that. Any advanced bookings have to be reconfirmed by 9am, I make it clear that I will not chase them to reconfirm and if they don't their booking will go to someone else. When they reconfirm I send them the parking postcode which within it states in capital letters PLEASE LET ME KNOW YOU GET THIS, if they don't acknowledge receipt of said text within 15 mins I send another one saying 'please let me know if you got the directions, if I don't hear back from you by ..... you're booking will be cancelled.. It has seemed to cut down the amount of no shows, although that can never be eradicated fully as there is always some arsehole that goes the whole hog.
I have had on occasion people reconfirm about 9.30 and act all indignant when told their booking is no more, but tough!