I don't do anything in particular.  Just go to the hairdresser as usual.  May want to bring your own products if you have preferences.  My mother brings her own each time and her hairdresser uses a range of great products.  Also, just be prepared for the heat (the hair dryer and blower) and do not be afraid to tell them to lower the heat.  High heat will yield beautiful results but not all people / hair can tolerate.  It's really a personal judgement.

I see.  You normally limit heat and only use blowouts mainly when stretching.  I still think the most effective way to prepare for a blowout is to make sure your hair is conditioned and maybe strenthened with either a protein treatment or reconstructor.  A heat protectant is also a good idea (but I do not use one).  I have been going to spanish beauty shops & getting blowouts for years and they always offer DT, Hot oil treatment or reconstructor.  They do not do any other type of prep.  I think these are most effective at preventing damage.  Also, stretch as long as is feasible but not so long as to cause more harm than good.  Sometimes I actually cause more damage from stretching too long than anything else.