You need to go to a shop that the owner or reception know you so well that they let you pay whenever you finish. I have three shops that I can do this and the owner tell the girl if I like it I will do 1 hour. There is a catch for this strategy.... if the girl doesn't want to do an hour with you, they will actually perform worse so you will finish in half an hour.
If you have to pay up first, you can check with the reception if you can extend to 1 hr within the first 10 mins. Bear in mind is an unofficial rule that if you extend at the end of the session, You will be charge another 30 minutes session instead of the 1 hour price.