The ancient Greeks were very concerned not to offend the gods. One thing that they believed was very offensive was to be ritually impure. Several precautions were taken to ensure that no impurities (miasma) were brought in contact with the temple or ritual space, or the participants in the rituals. Therefore, there are a few “rules” that you should follow in preparation for a Hellenic ritual..
Miasma can be broken down into two general types: birth and death. If anyone has recently given birth, or is in mourning, they cannot participate in the rituals unless they have undergone special purification rituals. Also, if you have killed anyone or anything “unjustly” you may not participate without purifications. (If you find yourself in one of these situations, please let the person conducting the ritual know beforehand.)
There are less serious types of miasma as well, and you can purify yourself from these with a ritual bath. Actually, we humans cannot be sure what of our normal daily activities might offend the gods, so we as a general rule always come to the rituals as pure and clean as we can be. Everyone is advised to shower/bathe before the ritual (in the morning is fine). Before entering the ritual space you should wash your hands in clean water, known as khernips.
One also must wear clean clothes. The style of clothes is not important, but “dressing up” just a little is preferable. Women should tie their hair back, as loose hair was a sign of mourning, and only permitted in the worship of a few gods. A garland on the head (stefanos) was always worn for rituals, by both men and women. This can be of flowers, leaves, or simply branches (real or artificial is fine). (You can see what one looks like on our page of photos.) If you do not have anything like that, a simple headband made from a ribbon, etc. is fine as well. Since sacrificial rituals were conducted outside, participants wore their regular shoes. However, some contemporary rituals are held inside, so be prepared to remove your shoes, and wear socks or bring slippers with you.
Some final things we should mention: many are surprised to learn that sexual activity must be kept away from the presence of the gods. Regular participants need not practice any abstinence, but if you do have sex before the ritual, you must shower/bathe before the start of the ritual. And a special note to the women attending: there were no restrictions placed on women due to pregnancy or menstruation, so you don’t need to do anything special for either of those circumstances.
If you follow these simple “rules” you need not worry about offending the gods with impurities.
For even more information, please see the essay about ritual paraphernalia.