Tipping at the hairdresser

Got a haircut on Wednesday, and was reminded again of what a delicate operation tipping is.
I actually sat in the car before going in and lined up my dollars - two bundles, one for the shampoo lady, one for the hair stylist. But when I should be tipping the shampoo lady is still a mystery, as it is quite difficult to clutch money in your hand as you recline for a shampoo, and although it was in my bag in my purse, it was in my bag in my purse, and she sends you off with dripping hair without any real opportunity to stop and rifle through the bag for the purse.
Key thing about tipping - it is meant to be sort of subtle - you act like you're not doing it, they act like they're surprised and pleased, rather than it sort of being expected. So, I think you're aiming to be just sort of slipping them some notes, and they sort of say thank you, but there is definitely no big conversation about it.
Back to the shampoo lady - this was the first time I had thought about it before hand, but I'm starting to wonder if I should have the money in my pocket ready to grasp as I sit up from the basin (rather than in my bag in my purse).
Then there is the hairdresser - they prefer the tips in cash, but are happy to put it on the card (but it creates complications when making sure each stylist gets her tips). I have seen one person just leave money on the bench below the mirror, but I decided to give it to her at the counter when I paid, which sort of worked, except that I'm still a little hazy on how much she is going to charge me, and the tip I think is meant to be about 15-20%, probably really 20%. I am not good at standing there at 7.30pm working out 20% in my head, without actually looking like I'm working out 20%. And, to be honest, I feel awkward slipping the money across the counter - I worry about the tip being too small, and looking ungrateful, as this stylist really is a good one for me, but I also worry about being too generous, and going broke trying to look like I have buckets of money and can tip generously all the time (I don't, I can't).
More complication - while I was having my shampoo done, one of the stylists gave the shampoo lady part of her tip, because the customer hadn't tipped the shampoo lady directly. So, could I just give one tip and assume the stylist will share?
I think tipping at the hairsalon is worse than anywhere else, because you have a personal, repeated relationship with this person.