1) Former NCNS.  

If someone has scheduled with me in the past and has a history of repeated cancellations at last moment or NCNS, they go on "deposit list", meaning I will not entertain them in the future unless they pre-pay.

2) Travel Dates

If I am traveling somewhere with a specific purpose of seeing one person, I do require 50% plus travel expenses.

You will hear as many "yes it is normal for a lady to ask for deposit" as "never ever make any deposits".

In the end of the day it all comes down to reputation.  For me one deposit represents less than a percent of my income .. so why bother ruining my rep over something like that?

Lina