For me it depends upon the circumstances.
If it's a short 1 hour incall I will book a girl via aw and if it falls through then I call another. If this doesn't work I often have a wank.
For longer punts where I may hire a venue, such as a dungeon /bdsm play space, I will book months in advance.
However recently I had booked and arranged a meeting with a girl who when I messaged her the week before we soon realised we'd had our wires crossed and it fell through. I put out the feelers on aw and did my homework here and managed to find a girl who I had seen before. The first girl to respond always gets the gig.
However work being what it is, and having to be flexible I have had punts where I have had last minute requests to be somewhere else and had to cancel. If this happens I give as much notice as possible.
Another scenario that I have experienced is where a punt is arranged a long time in advance and when I contact the girl the week before she doesn't answer. So I send her messages trying to keep the date, but if this fails I put out the feelers again and get someone else. I then cancel the first girl and do the punt. However recently I did this only to have the cancelled girl contact me saying she was ready for me to pick her up from the station and take her to the venue. Some girls should check their messages!