wiki answer, which agrees with what I was trained to do in the UK.
In rear facing seats, the attendant should be sitting with their back and head firmly against the back of the jumpseat, their knees and feet together and slightly in front of or behind the knee (depending on the individual airlines procedures) - commonly referred to as "toes to tail". In European carriers, the hands can be placed behind the head and hands one on top of the other and the elbows brought in to meet, taking care that the forearm does not cover the ear and restrict hearing. This position provides the flight attendant protection to the face from any flying debris (as it will impact their elbows) yet still provides them with the ability to view the cabin and not muffle their commands. In the United States, the FAA does not recommend placing the hands behind the neck as their research suggests such actions can cause unnecessary loading on the neck and spine during an impact.[1] Instead, US flight attendants are typically taught to sit on their hands, palms facing the ceiling, underneath their upper legs. Other variations include clasping the hands on the knees or using one arm to "hug" the opposing arm.
For forward facing jumpseats, the position is exactly the same but with the feet behind the knees, with some airlines requiring flight attendants to tuck their chin in to their chest ("bow to the captain") to reduce the likelihood of whiplash injuries.
There is also a third brace position for flight attendants, and that is the "normal" brace position. This is adopted by the attendant for every take off and landing and provides them with protection from any sudden emergencies and allows them to adopt the full brace position quickly should they need to. The only difference between the normal brace and the full brace position is that the attendants will either fold their arms across their stomach or immobilize them by placing their hands under their thighs with the palms up. This position forms part of every flight attendant's "sixty second review" - a technique being adopted by airlines whereby the attendant will go over various factors in their head during the take-off and landing sequence. Things such as "how do I open my door?", "where is the next nearest exit?", "am I over land or water?" and "what commands will I shout" are just a few of the questions an attendant will ask themselves. The belief is that this mental review focuses the attendant on the safety-critical role they have during take-off and landing and will result in faster decision making and adaptation to the scenario.