One of the biggest challenges to making a "cold shot" is that often the muscles I use for properly drawing my bow are stiff. The simplest way to cure this is to periodically pick a target, draw your bow, aim, hold, and let down your draw. This keeps you loosened up, plus drawing and aiming without actually shooting helps you focus on the target.
Although just drawing and aiming will help a lot, the single biggest help is to actually shoot while out hunting. A common practice among traditional shooters is to carry one or two blunt pointed arrows in your quiver so that you can stump shoot in your down time. Stump shooting is fantastic for keeping you warmed up, but unlike just drawing, actually completing your shots will bring your release into play, as well as give you all sorts of angles and situations to practice.