I've never booked anything more than a week in advance. One of the reasons that providing is an attractive occupation is that a lady can make her own schedule and pretty much work only when she wants to.
It's also a good idea to have a back-up plan. If I'm looking for an appointment while traveling I always try to find at least two or three possibilities on TER and then start by trying to book with the lady that would be my first choice.
Don't be afraid to move on down the list if a lady seems too difficult, or just too busy, to get an appointment with, I've met some really dynamite ladies when forced to go with my second or third choice....you never know where your next great experience may come from.

if I plan to travel to another city on some business.

Usually works out, there have been or two times that the provider had to cancell for some emergency, but that could just as well had happened if I had called the day before.

Stay in contact with the provider every few weeks via email.

The better providers seem to appreciate someone who plans ahead.

Just remember the further in advance you make an appointment the more you open the possibiity of a cancellation just because "things" happen.

If traveling, try at least two weeks before to give the provider a chance to verify you, schedule you etc.

If you are staying on your home turf so to speak, a week or less before works.