Monastics in the Theravada tradition have chosen a way of life that may seem different to most westerners not accustomed to it. A few simple guidelines are all that are needed to help you feel comfortable in supporting their practice.
The most commonly used expression of body language in the monastery is the añjali. The hands are held palm-to-palm in front of the heart and are sometimes raised to the lowered forehead. It is a gesture of respect that can be used as a greeting, a goodbye, a thank-you or when speaking with one of the monastics. It is sometimes accompanied by a slight bow. Nuns and monks can have no physical contact with anyone of the opposite sex. This includes shaking hands so the añjali is used instead.
During meditation and dharma talk, it is traditionally considered impolite to point one's feet at either at the monks and nuns or statues of the Buddha. Also, lying down or stretching out is considered inappropriate in the meditation hall. While sitting, care should be taken to move and shift positions quietly. It is also traditional to remove head coverings.
As alms-mendicants, monastics are prohibited from engaging in activities that could provide for their own material livelihood. This includes handling money, cultivating crops, and working the land or storing food. As a result everything that accrues to the monastic community is the result of an offering from a generous person. Anything a monastic consumes, except water, must actually be offered to them directly. They cannot help themselves to food unless it has been given to them. They also take no food after mid-day.
In addressing a monk or nun, it is considered impolite to refer to them directly by name without an appropriate form of address. Any monastic of more than ten years standing is usually addressed as "Ajahn", which comes from the Thai and means "teacher".