I am posting from New York but I have to assume this is a general problem and it's something to consider as we all head towards the busy fall and into holiday season.
It is BEYOND helpful when you call and let a restaurant know you cannot make your reservation.
Even if it's 8:15 and you were booked at 8 it's not too late. Besides being very good manners doing so is beneficial to all in the chowhound community. It increases you chances when stopping by an establishment just to see if you could get a table.It increases the chances of a reservationist giving you a booking or calling you off of a wait list. And if everybody did this it would decrease the number of "impossible to get reservation restaurants."