The identification of a species that is new to you is a complicated process, as many of you now know. Sometimes you find that a species is unequivocally identifiable based on a unique character that is not shared by any other species in the region. Other times you go on gestalt and deduction, reckoning that such-and-such a set of characteristics on the animal or plant you see could only be held by this particular species, but no single unequivocal character exists. Sometimes you accept the identification because you are swayed by the experience of a more knowledgeable colleague and base your identification solely upon his or her lofty proclamation (but hopefully not very frequently!).
As one continues in studies of natural history, eventually one comes to identify a number of species that are actually quite similar. Sometimes, especially if you haven't been in the field for a while, it may be difficult to recall the character(s) that led you to your diagnosis. If you are to learn to recognize a species (or a genus or other level of taxonomic organization) without going through the entire process of identification upon each encounter, it will be useful to you to internalize and form a mental picture of the characters that led you to your diagnosis. Drawing and sketching key characters of the species (genus, etc.) directly from a specimen is an excellent method for forming a mental association between the form of the organism and its name, thus expanding your natural history vocabulary.
The objective of this exercise is to get you to look closely at an organism you have identified and form a direct relationship with it, and use drawing or sketching to record those aspects of the organism that speak to you of its identity. The emphasis of the exercise is on looking closely at an organism and asking the question: "What is it that makes me believe you are (a member of such and such taxonomic group)? For example, you find a branch of salal. What particular aspects of this branch tell you that it's salal? Is it the shape of the leaf? Its venation? The form of the leaf margin? The leaf's size? The arrangement of the leaves on the branch? In one or more drawings in the journal or species account section make a neat sketch of each character that says to you "I am salal". It doesn't matter so much what the drawing turns out to look like; what matters is that you draw the character from the specimen. You can do it as quickly or as slowly as you need to. The idea is that later, when you look at the drawing, you'll be able to envision looking closely at the individual. You'll know that you were successful if later you can use the drawing to recollect how you identified the specimen and confirm the identity of a new individual, or confirm that the specimen is of a group you haven't seen before!
A couple more examples are appropriate. You identify a frog as Ascaphus truei, the tailed frog. Fortunately, it's easy because it's a male. You begin a species account and you draw the intromittent organ from a ventral view. Later, you find Ensatina escholtzi and you draw the constriction of the tail and indicate the brown colored areas of the limbs. You measure the individual and record the size and the number of costal folds. Or, for example, you find red alder cones and old leaves below a tree. You draw the leaf, paying special attention to its margin, the cone (paying attention to the number of rows of bracts and it's size), and a sketch of an individual seed using your lupe (hand lens).
Find at least three different species, identify them. Then, draw the characters that spoke to you of the individuals' identities from the organisms, not a book. Indicate size by including a scale bar. An excellent natural history journal will have one or more drawings for nearly every new organism that is identified from now on. This is one strong indication that you have conducted careful observation.