What follows is a quick summary of the compulsory process for Texas defendants, taken from Clark v. Texas, 14th District Court of Appeals (Houston):
Under both the United States Constitution and the Texas Constitution, a defendant has a right to compulsory process in order to call witnesses to testify on his behalf. U.S. CONST. amend. VI; TEX. CONST. art. I, § 10; Etheridge v. State, 903 S.W.2d 1, 7 (Tex. Crim. App. 1994). The Texas Code of Criminal Procedure, however, requires a defendant to file an application for a subpoena with the trial court‘s clerk if the defendant wants to ensure the witness‘s presence. Tex. Crim. Proc. Code Ann. § 24.03(a) (Vernon 2009). The defendant must then properly serve the witness. Id. § 24.04(a) (Vernon 2009). If the witness ignores the ―duly served subpoena, the requesting party shall be entitled to a writ of attachment ―commanding some peace officer to take the body of a witness and bring him before such court . . . on the day named . . . to testify in behalf of the State or of the defendant . . . . Id. § 24.11 (Vernon 2009); see id. § 24.12 (Vernon 2009); see also Rodela v. State, 829 S.W.2d 845, 848 (Tex. App.—Houston [1st Dist.] 1992, pet. ref‘d).
A defendant is not entitled to a writ of attachment if he fails to properly serve the subpoena on the witness. Ford v. State, 14 S.W.3d 382, 391−92 (Tex. App.—Houston [14th Dist.] 2000, no pet.); see Erwin v. State, 729 S.W.2d 709, 713−14 (Tex. Crim. App. 1987), overruled on other grounds, Burks v. State, 876 S.W.2d 877 (Tex. Crim. App. 1994). If a party duly subpoenas a witness and the witness does not appear, the party must follow a three-step process to preserve error—the Erwin procedure. Sturgeon v. State, 106 S.W.3d 81, 85 (Tex. Crim. App. 2003). The Erwin procedure includes: (1) seeking a writ of attachment, which the trial court must deny; (2) showing the court what the witness would have testified to; and (3) demonstrating that the witness‘s testimony would have been relevant and material. Sturgeon, 106 S.W.3d at 85 (citing Erwin, 729 S.W.2d at 714).
As a practical matter, I would argue that you should serve subpoenas on all of your critical witnesses. That way, the State is in charge of seeing to it that they appear for trial. Once you've served your subpoenas be careful to follow the proper procedure to preserve error, because the appellate courts will be all too willing to find waiver where possible.