Loose clothes and fabrics that stretch are best for an AT lesson. Alexander Technique does not demand athletic movement, but it requires that your head, neck, limbs and joints be free. You may find it helpful to take off your shoes, and you will need to remove your hat, or any other head coverings - your teacher's hands must be able to make contact with your head.
You will spend some of your time lying down on a table with your knees up, so if you wear a dress or skirt, wear leggings for modesty and freedom.
Be prepared to stretch your mind. Alexander Technique really isn't quite like anything else. To fully appreciate how it works, you will need to open yourself to new experiences and not assume that AT can be understood by direct comparison with something you are already familiar with (Feldenkreis, yoga, massage, chiropractic treatment, meditation, or other modalities).
You will need to put aside the demands and distractions of everyday life for about 45 minutes while the lesson is being conducted. Turn your cell phone off. You will also need to repeat this quieting process for two fifteen- to twenty-minute "lie-down" sessions per day (your homework).
In addition to the twice-daily lie-down sessions, you'll be applying your new Alexander skills in the course of your own everyday activities whenever possible.
Bring back questions from your life experiences with AT to ask your teacher. Experiment. It never gets boring, and the more willing you are to try things out, in lessons and in life, the more you will learn and enjoy your new discoveries and abilities.