Start with the 10% rule. Depending on the procedure, a 10% gratuity is pretty standard, and you can exceed as appropriate. To reiterate, most spa workers view 10% as a minimum amount. Anything less puts you in asshole territory. As for the heights of your generosity, “It’s the same as being a waiter,” Scrivens says. “A 20% tip tells me I’m definitely doing everything right."

Then consider the service. Tipping in the spa industry tends to vary by the difficulty of the service. Using 10% as the standard, manicurists are often tipped a little less because they don’t take as much time. Facials demand a little more moolah because they take more energy from the practitioner.

Don't bankrupt yourself. At Azure Day Spa (26 W 20th St between Fifth and Sixth Aves, 212-563-5365), some types of laser skin treatments can run $1,500 to $2,000, so even a 10% tip can be awfully steep. “That’s a judgment call,” says owner Mina Bowker. Customers of these high-end services may keep their appreciation in the still-healthy $50 to $100 range, and no one is insulted.

Make a smooth exchange. How you handle the actual tip is a style thing, pros say. You can leave it in the room. You can try to do the sneaky handshake thing and palm it off. If you want to stick to established norms, spa owners say most customers slip it into an envelope kept at the front desk. (Make sure you write the service provider's name on the front.) Regardless of your donation method, two specific habits will make you stand out and enjoy better treatment next time. If you enjoyed your experience, make a point to shake your practitioner's hand and offer a sincere “thank you.” (You may leave the tip at this point or not.) Secondly, though no one will complain about a credit-card gratuity, spa workers definitely prefer cash. That keeps your compliment private and out of view of Uncle Sam.