For hair, paying a visit to Nadia is a sure bet, she's an Italian that's been on the island for several years, and is a wizard at making your hair look great again, restoring it's luster after it's been abused by the sun and salt water. She's located in Alba Plaza between Sandy Bay and West End- it's a bit far of a walk, better to hop in a bus for $1. She gets busy, so contact her for an appointment ahead of time: https://www.facebook.com/bornatinadia
Need a wax or nails done? Sophiestication is on the 3rd floor in Plaza Mar in Coxen Hole, and they have several staff, so can usually see you if you just walk in.
If a massage is what you're looking for, there are plenty of massage ladies on the beach in West Bay, though you might find they do more of a body rub than a real massage. There are a couple of great massage therapists that do housecalls, like Doris and Johel- if you need their contact info, just let me know. There are also several spas on the island, such as Baan Suerte in Sandy Bay, Anthony's Key also has a spa in Sandy Bay, and Serenity Day Spa in West Bay, though pricing may be higher than backpackers are able to fit into their budget.