Anxiety Disorders:
This problem might be conceived from an attentional standpoint as focusing one's attention on worrying about the future. Persons with anxiety disorders are rarely distressed by current events, but rather a person's concerns about what catastrophes might occur. That is, they are continually directing their attention to things that are not, rather than things that are.
Clearly, Zen meditation is helpful for this. Completely absorbing oneself in environmental sensation (visual, auditory and kinesthetic {breath and touch}) instead of focus on conceptual worry initially points out the extent to which one is not "present" and then trains one to attend more to current circumstances.
Another problem is that persons with GAD and panic disorder have significant somatic arousal. Tai Chi and Chi Kung are superb for both moderating somatic and attentional complaints.
GAD: Active meditations should preceed more passive meditations. Therefore, Yoga or Tai Chi is useful before Zen meditation is attempted.
Panic: Gaining control over what is experienced as uncontrollable thoughts, emotions and physiological reactions is uppermost in patients with panic. Therefore, teaching patients to exacerbate their symptoms slightly, and then reduce them can be helpful - both prior to and during the onset of panic attacks. Yoga training can be extremely beneficial for this, in addition to cognitive therapy to explore fears of death or immanent demise.
PTSD/Acute Stress Disorder: PTSD and ASD have both dissociative and hypervigilant dimensions. Those who have poor ego strength have cognitive and affective flooding, for which Zen Meditation is extremely helpful. Those who are more dissociative do not understand why they react so strongly to certain triggers (colors, persons, places) - only that they become filled with rage or fear. For these patients, a reflective approach to meditation, such as Vipassana/Mindfulness can be helpful. Here, they comment on what they are noticing, which minimizes their reactivity while maximizing healthy reflection. Thus, those with "flooding" of thoughts and feelings can develop better ego strength, while avoidant/dissociative types will develop the ability to allow "controlled abreaction."
Specific Phobias: Exposure therapy (imagery of a phobic object/event during relaxation) has been proven effective for many patients. Meditation of any form is helpful in sensitizing patients to cognitive, affective, physiological and behavioral reactivity in phobic situations. Further, meditation can help reduce reactivity and help patients focus on a more comforting alternative (e.g. the breath).
I find that before a patient is willing to regularly practice meditation, they must understand that their worry is excessive and harmful to them. Therefore, meditation is a useful adjunct to cognitive therapy (and visa versa).