For Spas:
Aquavita Medical Spa -- I haven't been but a friend went and she said it was ne of the best massages she's had here. They are in Recoleta approx. Arenales & Callao
Another friend just went to Evian Spa on Monday and he said the service was great -- both of these are listed in the Time Out Buenos Aires Guide. I think they are somewhere near the Jumbo near to Las Canitas -- I think they should come up on google.
For salons --
Staff Cerini was fantastic, you don't need an appointment -- there's about 20-30 colourists and then stylists etc, you can also get manicures and pedicures. They were able to match my colour perfectly with what I get at home. I believe their website is staffcerini.org, but I think they are also in the TimeOut -- they are around Montevideo or Uruguay & Marcelo T Alvear.
Another hair salon that is supposed to be excellent is Barcelona, and yet another is New Station.
The best pedicure I've had yet was at Nail Designers -- they have multiple locations.
Have fun!