
Nothing beats having the pups booked before they are born which you can achieve by advertising the planned mating long in advance (I would normally start a year in advance, at least), and obviously using really good quality parents with a good record from shows or any form of working, so that people are prepared to go on a waiting list for a quality pup. I find the people prepared to wait are almost always the best buyers, as they are the ones that have planned ahead and are not making rush decisions. Of course, you almost always end up with some (or many) people pulling out, but those that stay on a waiting list tend to be well worth it. I had 11 pups wanted from my last litter and 7 pups born, so could take my pick. For my next planned large breed litter I currently also have 11 pups wanted, and the bitch isn't even in season yet.
When it comes to having to advertise pups already born, I find it is a lot harder -but of course it can vary from breed to breed. At times when I have been desperate (such as the time when I had NINE people pull out after the bitch had been mated) I have advertised everywhere, good and bad places -but I only got decent responses from CD and the KC plus above all my own website. The dodgy sites brought either no enquiries at all or dodgy ones.
Then of course there is your breed club's puppy list -most breed clubs tend to have puppy co-ordinators.
As for what to ask of potential buyers, I find the best approach is often to ask people to tell me about themselves and in particular about any dogs they already have or have had before, and explain what got them interested in my breed. When people tell you about other dogs they have had it seems to give a lot more than if you just ask questions, as you can tell quite easily if they had a much loved dog or not. If they have never had a dog before then for my breed I would not let them have a pup as my main breed is not for novices, for easier breeds I suppose it comes down to asking more questions. Also I find what works better than anything else is making sure to tell them about your breed's BAD points as well as the good ones. If people are not put off by destructive, energetic, noisy dogs that need a lot of training, then they are usually good ones. :)
And finally, another reason for preferring to have a waiting list is that people can come and visit you BEFORE you have puppies. Walking in here they will be met by 7 little and 4 large dogs (the rest of mine do not mix with that lot and tend to spend their days outside so are not near the front door) that bark and jump and end up on people's laps, and how they deal with THAT also says a LOT I find. :)