Some general thoughts. Your mileage may vary - some of these might work well for you, some probably won't work at all. I'm not in business, though I've had a few narrow escapes, but I've worked as a sales person for small and medium sized businesses for a few years now.
First: if there has been no exchange of money, they are not on your calendar. Not that you can't inform someone about what your current availability is! But until they put down deposit, you aren't scheduling them, and that slot you had free last week could just as easily go to someone else (or your own project, or your vacation). Be upfront, calm, and clear about this - it's not some kind of pushy tactic, it's a very common and reasonable business practice, and it will encourage people who are serious to get on the ball. In my current job we say "...but we are not holding space for you until after deposit." Especially if someone acts like everything is all ready to go and when is the project starting? when you haven't got any money, be very clear and firm that everything is not ready to go and the project is not starting until dollars.
Following on that: Charge deposit. Pick a convenient number that covers your starting costs and will mitigate the blow if someone backs out. Make it nonrefundable. Make it big enough that someone wouldn't want to just walk away, but reasonable given that you haven't actually done any work yet. Charge extra if the start costs are going to be extra big, or if they're demanding a huge commitment of time or energy. Don't justify any more than you have to: "This project needs to be done in two weeks and will require ordering large quantities of expensive things. Your deposit will be my usual $X, plus $Y rush fee and $Z materials fee. It is nonrefundable, and I will not be able to start without it."
Ok, it's bedtime and besides I'll probably be a lot more coherent in the morning. I'm going to post this so my login cookie doesn't expire, but I'm leaving the window open and I'll come back and add more later. Ta!