I had a request for this and I think this would be a great education post. First, if you are interested in harvesting plants from the wild I recommend investing some money in a field guide for your area of the country. These are easily found on the internet and local bookstores and will only cost $10 or so. They have pictures plus information about uses for plants and the ranges they can be found. Plus you can take it with you so you can compare a plant with a picture and make sure you have the right plant! Many people have died from mistaken identity! It also pays to research individual plants. I'm going to give some general guidelines but reality is going to be different depending on what plant you're dealing with.
-When you go wildcrafting make sure you don't trespass on someone else's property. You may actually get enthusiastic permission to harvest plants that are considered a "problem" like stinging nettles.
-Don't take every plant you find in a wild stand. If you run across a good patch of yarrows don't pull them all up and take them home. Only take what you need so you don't waste the plant, and to allow it to reseed itself. This will insure that you have plants to harvest the next season. It is generally recommended that you take no more than 10%. Here's where it pays to do your research. If you are harvesting dandelion, chickweed, cleavers, etc. I say take all you want. If you are out looking for ginseng, goldenseal, black cohosh, etc. don't take any. Learn the status of the plants you find and if it is even legal to take it!
-Make sure areas that you wildcraft from aren't polluted or treated with chemicals.
-Learn when you should harvest a particular plant (or plant part). Some plants should be gathered while in flower, some before or after flowering. Roots should be harvested in Fall after the foliage starts to die off or in very early Spring before the new foliage or blooms begin.
-As soon as you get your plants home they should be shade dried or made into whatever medecine you are making. You want to preserve your bounty and not let it lie around to rot.
Feel free to post comments and questions. I do look at them I will get back to you.