What constitutes reasonable diligence in procuring a witness's presence at trial?

Under Evidence Code section 240, a witness is unavailable when he or she is "[a]bsent from the hearing and the proponent of his or her statement has exercised reasonable diligence but has been unable to procure his or her attendance by the court's process." Prior testimony of such a witness, although hearsay, may be admissible under Evidence Code section 1291. The question raised by defendant is whether the prosecution exercised the reasonable diligence (sometimes called "due diligence") required by Evidence Code section 240. However, the question of reasonable diligence is a factual one and will not be disturbed on appeal unless there has been an abuse of discretion by the trial court. (People v. Enriquez (1977) 19 Cal.3d 221, 235, 137 Cal.Rptr. 171, 561 P.2d 261.)

What constitutes reasonable diligence in procuring a witness's presence at trial?