First the herb must always be taken with the left hand. This is because the left hand is receptive. When the power of an herb is harvested it is received and not taken.
Second, the wind (if any) must be at your back when the herb is gathered. This is because the wind indicates the presence of spirits and to have them support you from behind is beneficial. To have them pushing against your movements is detrimental and you risk offending them. In other words you are either with the flow or against the flow.
Third, you must never look back over your shoulder. This is because you might scare off the fairy folk and other nature spirits who have gathered to take witness. Thus abandoned, you risk negative charges on the herb.
The fourth step in harvesting an herb is to trace a circle around the herb with your magickal blade. This prepares the spirit of the herb for withdrawing and ensures that it stays with the herb when harvested. It is essential that iron never come in contact with the herb or touch inside the circle at any time. Iron negates magickal magnetic charges.
The fifth step requires that you speak to the herb telling it why you need its help and what you are about to do to it.
The sixth step is to place the herb in a pouch, never letting it touch the soil. If it touches the soil the spirit will pass back into the soil.
The seventh step is to wear no jewelry or clothing and to have abstained from sexual intercourse for seven days (a lunar quarter). This helps to magnetize your aura, and being nude while you gather the herb makes you a creature of Nature again free from the signs of domestication.
The eighth step is to leave a small gift or offering in the hole from where the herb was withdrawn. The traditional offering is a mixture of equal parts of wine, honey, and milk. In place of this, a silver or copper coin may be planted as a gift to the Earth spirit. Perhaps the best offering might well be a new herbal seed.
The ninth and final step is to kiss your hand to the moon as a token of love and respect.
When taking an herb in this manner you will want to decide in advance whether you need the entire plant or simply a leaf or two. The leaves will only provide you with pharmaceutical ingredients, as would only the roots or stems. To obtain the spirit of the plant you must harvest the entire plant intact. If you harvest only the leaves of an herb then avoid pouring the libation directly upon the plant. If using a coin simply press it into the soil nearby.
If harvesting wild plants leave a large amount of flowers, seed and root as the plant population of that area will very quickly die out if you go in mob-handed and wrench up the only two plants for miles around.
Source: Wiccan Magick