
How can you have a 'future litter' - my best way was to kekep a note of enquiries in my puppy enquiry notebook. It seems that people are advertising, with a price yet, on another well-known website, BEFORE the litter is on the ground.
For me, there are normally 2 reasons for breeding - the BEST being when the breeder needs a new generation. The second being once the notebook is reasonably full!!
People can add their name to a waiting list on here, but I don't know about breeders doing what you want.