What is a common technique used for locating difficult defendants?

The correct choice, skip tracing, refers to a systematic process used to locate individuals who are hard to find, often for legal or financial reasons. This technique is particularly useful in the context of private process serving, where locating defendants who are evading service is critical.

Skip tracing involves various methods, including searching public records, databases, and social media, as well as utilizing information from acquaintances, former addresses, and other resources that might provide clues about the individual's whereabouts. This practice can be very effective for process servers who need to serve legal documents to individuals who may be deliberately avoiding being found.

In contrast, legal representation, document filing, and negotiation do not primarily focus on locating individuals. Legal representation pertains to advising and representing clients in legal matters, document filing involves submitting legal documents to the court, and negotiation involves discussions aimed at reaching an agreement. These activities are not specifically aimed at tracking down difficult defendants, highlighting why skip tracing is the appropriate answer for this question.