Lynn Massey, an LMSW skilled in brief ED interventions with substance abusers, explained many important characteristics of MI in a recent workshop I attended. In addition to well-known approaches such as empathy and collaboration with patients, she emphasized the importance of evocation (eliciting, accepting, and understanding patients' ideas about behavior change) and respecting patient autonomy, because in the end, it is the patient's choice to change a behavior or not.
It's also important to realize that motivation, or readiness for change, is a very dynamic process that is the deciding factor when it comes to patient adherence to treatment plans, behavior change, etc. In fact, there is a model for understanding the process of change that can be very helpful for the frustrated clinician:
source: adultmeducation.com