Wow Sharon, what excellent resources! I'm bookmarking this page for myself, as a few of them were new to me
I just want to echo what Sharon said. Even though you are "only" the stud dog owner, you are still partly responsible for those puppies.
A few things you should think about and know your own answers to before you agree to any breedings:
What faults does your boy have? You will not want to breed to a bitch with the same faults. You'll also want to keep track of the pups, so that you can learn what qualities he passes on to his offspring - and how much influence the bitch has over those qualities. (For example, my brood bitch is a little bit straight in the rear. For a stud dog, we wouldn't want to use a dog that also is straight in the rear - except, as was the case in her first breeding, when that stud dog *produces* dogs with ample rear angles.) That way you'll be able to help the bitch owners when they come to you - he won't be right for every bitch, and they'll respect and appreciate you telling them why he isn't.
Also - what will you do in the event of white puppies? Do you plan to follow the ABC Code of Ethics (which you must do if you ever want to join even a local Boxer Club), which forbids the registration or inclusion in the litter count of any white puppies? Make sure, if this is the case, that it is spelled out in the stud service contract. Since you have to sign the litter application, you can put that you will not sign any litter application that includes in the count puppies of a color not allowed in the breed Standard (or however that's phrased in the COE).
Know what type of guarantee you are going to offer - what do you consider a successful breeding? Typically around here it's two live colored puppies that survive past seven days of age. If the breeding doesn't take, if there is only one puppy - or if they are all whites - what recourse will the bitch owner have? Will you refund the stud fee? Offer a free breeding the next time she comes into season (or on another bitch of similar quality)?
Are you equipped to house the bitches? Typically the bitch will come to the stud. You'll be responsible for her care while she is there to be bred (and for making sure no other dogs have the opportunity to breed her). What happens in the event of a veterinary emergency? Who pays the costs? That contingency should be covered in your contract as well.
Do you know how to do a breeding? It isn't always as easy as putting them together (usually it's only that easy when you *don't* want that particular dog and bitch bred

) - especially if the bitch is a maiden. Do you have a mentor who will help you with the first few breedings, until you're comfortable handling them on your own?
What about the puppies? Are you going to request names/addresses of puppy buyers? Require that they be sold after a certain age - and that they not be sold to any brokers or pet stores? Are you willing to help with placement of the puppies (at any time during their life)?
A couple more good links for you:
http://clubs.akc.org/vizsla/studdog.pdf
http://www.bluegrace.com/virtualbreeding.html
The Virtual Breeding site focuses primarily on the bitch, but there is a brief look at what can happen to a stud dog - and IMO anyone who is breeding, bitch or stud owner, should know what is involved in the entire process - and what can go wrong.