I have a no tolerance policy for this. Until you give me a non-refundable deposit, it is just talk and you are not on my calendar.
I use PayPal Invoice, as well as accept cash & check in person. Although, nothing is in stone until the check clears. I am busy and have plenty of people who want the same date. First to pay gets it. All forms of payment receive a receipt as well.
If someone expresses interest in a certain date/time and SAY they want to pay the deposit. I hold their date for 24hrs to give them time to actually pay it.
It's disrespectful for you to waste MY time and think that you are so important that my world revolves around you. Don't jerk me around and waste my time. Pay your deposit or keep moving. I won't waste your time, so don't waste mine. You wasting my time, costs me money. If you are set on having me do your photos, you are going to have to pay, so what is the problem in paying a non-refundable deposit now?
For your particular case, it sounds like you have given her ample opportunity to pay and reserve her time on your calendar. Since she is communicating with you, it sounds like she is still planning on it. Tell her, "I need you to pay your deposit now or I will be forced to remove you from my calendar." If she doesn't pay, she knows what happens. I would keep her on my calendar but mark it as un-paid and consider it an open time slot. If someone else is willing to pay the deposit, schedule them in and no need to notify her that she is not scheduled anymore because you have already told her several times and made it clear. When/if the time comes and she contacts you angry about it, refer her back to the reminders and warnings you gave her.
I have had several potential clients email me and ask to book me for a certain date/time then never pay a deposit. So, I did just what I told you to do. I scheduled someone over them who paid their deposit and did not notify the previous "client." And guess what, I never heard from them! They never called to cancel or asked where I was. Talk about disrespectful! They said "put me down for this date/time" then at some point decided NOT to do photos and did not think it would be appropriate to inform me?! pft!