How to Take Tissue Samples and Use the Results- Dr. Steve's ATW Ep 27

What is the correct process to take a plant sample?  We talk about how, when, where, what part of the plant and common mistakes that can be avoided.  Plant analysis can be an effective way to understand many nutrients and the ratios of those nutrients.  Many times these deficiencies can be detected before the plant shows visual signs.  However there are some nutrient analysis that are not as reliable.  Which ones are they?  What is the best way to identify these deficiencies?