I'm afraid you can't be nice, you'll end up loosing all of the bookings. No-one who I asked to wait while someone else dicked about ever did. I just treat every date as available until someone has paid me for it, simplifies the whole moral dilemma.
Why are you posting out a form? Email a pdf or word, then they can complete it and scan and email it back. Or send it back if they must. Put into your letter that until you have received the deposit payment and the booking form the reservation remains open to others - or something similar.
So far this year I have not had to do any contracts which was not the case last year with the French.
I use PIMS and have now set up a French booking form with help from e-Richard and Martin (dmj) and so far so good.