Yep, I agree with Lil' Lori- get your appointment right away the day you board! They fill up very fast for formal nights, especially when you want a time slot relatively close to dinner.
I got an updo done on the Mariner in April for our first formal night of the trip. I paid $45, which is the shampoo and style fee. I have to admit I was not super impressed with the salon staff onboard, but my hair came out decent and it was a fun experience so I really can't complain (I'm just REALLY picky- my sister is an upscale stylist and no one else touches my hair usually!)!
Majesty OTS, 4/07
Mariner OTS, 4/08
Adventure OTS, 4/09