First, study the poisonous plants so that you know before picking how those plants might look like.
Only pick the species you know that are common in the area. Look out at places where pesticides are used or other human contamination.
It is certainly important to find out which part of the plant you can pick and when. Some plants develop toxic substances as they grow (such as ferns, celandine, ...). Other species are best picked after a night of frost (such as blackthorn). It is better not to drain moisture from a birch when the tree develops leaves because otherwise you will deplete the tree.
Only pick what you need for your own use. Do not pick everything in one place or on one plant. Leave enough for nature so you don't disturb the ecosystem.
Leave no trace of your visit.
Wash all plants thoroughly to remove any parasites. When cooking the plants above 60 degrees any unwanted parasites eggs also die.
Be sure to check the preparation method of the wild plants to know how to defuse dangerous substances, if there are any.
LEAVES: SPRING - SUMMER
Because of their aniseed-like taste, they are great for salads. Coltsfoot is a popular means of respiratory treatment. Meanwhile, it has been discovered that the plant contains harmful pyrrolizidine alkaloids, so daily consumption is not recommended.
Lot of wild herbs and flowers could be indeed poisonous, containing unexpected human contamination or parasites. It's important to look out to avoid this. Although let not forget that many vegetables and fruit we buy were also sprayed with pesticides. And certain drinks - food might be more dangerous for body and soul, than a small amount of certain poisonous plants.
Some interesting books and websites
Plantes aromatique et culinaires by Jan Kybal
Das Grosse Handbuch der Kräuter und Heilpflanzen various authors
Wild plukken by Leoniek Bontje and Yvet Noordermeer
Naturalmanak by Claus Meyer
Welche essbare wildpflanze ist das? Kosmos - Naturfuhrer