For collecting evidence: A set of good eyes, and attention to detail. I'd say everything matters while collecting and analyzing a crime scene. What you see, how you see it, what motes are you taking, does it make sense, what are you collecting , what are you processing on site and what can you take from the crime scene, photographs... before during and after every procedure and lastly what process suits the item the best. But then again I am just a student I do not know if you mean something more technical? Sorry I am new.
Sincerely L.E. Cardinal