Q:

Evidence-based best practices for writing a gratitude journal

I was intrigued to read (in the question "What positive writing exercises improve happiness?") the idea of a gratitude diary suggested as an intervention that "causes psychological well-being levels to increase in a lasting way".
Empirical studies suggest that people who use gratitude journals feel better about their lives and report fewer symptoms of illness. (Emmons & McCullough, 2003; Doverspike; Emmons Lab.)
However, there's also lots of dubious law of attraction-style writing on the topic, like this:

"Did you know that appreciation, gratitude and love are the highest
  forms of vibration? You can only have one vibration at a time, and if
  you are noticing what you appreciate and noticing what you are
  grateful for, you can't be noticing what you don't like."

And although studies find gratitude journals to be beneficial, they disagree on the most beneficial ways to keep one. Psychologist William Doverspike says:

"A daily gratitude intervention (self-guided exercises) resulted in more positive effects tha[n] did the weekly intervention."

But Jason Marsh of the Greater Good Science Center at the University of California, Berkeley says:

"Writing occasionally (once or twice per week) is more beneficial than daily journaling. In fact, one study by psychologist Sonja Lyubomirsky and her colleagues found that people who wrote in their gratitude journals once a week for six weeks reported boosts in happiness afterward; people who wrote three times per week didn’t."

So: which is likely to be correct in practice? And what best practices for keeping a gratitude journal can be inferred from other research, either specifically about gratitude or more generally in the cognitive sciences?
References:

Doverspike, Ph.D., William F. "Gratitude: A Key to Happiness". Georgia Psychological Association.
Emmons, R. A. & McCullough, M. E. (2003). Counting blessings versus burdens: An experimental investigation of gratitude and subjective well-being in daily life. Journal of Personality and Social Psychology, 84, 377-389.
"Gratitude and Well-Being". Emmons Lab at the University of California, Davis.

Update (9th Dec 2013):
I'm continuing to research this question. This article, while not terribly scientific, has some good jumping off points (infographic and links) that may help potential answerers!

A:

Empirically supported treatments are specific interventions which controlled (generally quantitative) research has demonstrated to be effective for specific populations (http://en.wikipedia.org/wiki/Evidence-based_practice). The empirical literature on gratitude is rapidly expanding, but, as many of the reviews I will mention note, still emerging. Several recent reviews and meta-analyses address the support for gratitude interventions, including journaling. A 2010 dissertation reviewed 12 gratitude studies and found that letter-writing and list-making (e.g. gratitude journals) were associated with positive psychological outcomes (Borgueta). A 2010 narrative review also looks at 12 gratitude studies, and reports that while most studies find positive results, methodological limitations make it difficult to draw firm conclusions (Wood, Froh, & Geraghty). 
A 2009 meta-analysis of 51 studies assessing the impact of positive psychology interventions on depression found small to moderate effects (Sin & Lyubomirsky). That meta-analyses included 20 studies with gratitude or positive writing interventions, and provides a discussion of possible moderators. A 2013 meta-analysis of 39 positive psychology studies found smaller, but still significant, effects for similar interventions on psychological well-being and depression, some of which remained at 3-6 month follow up (Bolier, L. et al). This analysis included at least 12 studies that used gratitude or positive writing interventions. 
These studies do note that the interventions are not effective for everyone and discuss some factors (gender, positive affect, length, etc) which may impact receptiveness to and effectiveness of these interventions (Parks, Schueller, & Tasimi, in press). Clear demographic and situational moderators are only beginning to emerge, but these reviews may help you determine the clients for whom these interventions may be most effective. In general, although some studies did not find the interventions to be effective, there is little mention of the interventions being harmful. Additionally, the interventions tend to be easily understood and implemented, requiring few materials and a brief time commitment; thus, they may be worth trying, even if a study has not been conducted on a sample with demographics that precisely matches your clients. 
These studies list a range of possible gratitude interventions and the current evidence for each approach. As with most therapeutic interventions, the answers here are not black and white. Practicing evidence-based care involves not only identifying the best available support for those treatments, but then using clinical judgment to determine whether it would be worthwhile to implement the approach with a particular client, given the client’s individual characteristics and the therapist’s competence in providing the intervention (http://www.asha.org/members/ebp/). Hopefully these resources will provide some support for that decision.
Bolier, L., Haverman, M., Westerhof, G. J., Riper, H., Smit, F., & Bohlmeijer, E. (2013). Positive psychology interventions: a meta-analysis of randomized controlled studies. BMC Public Health, 13, 119-129.
Borgueta, A. M. (2010). Adapting gratitude interventions to the practice of clinical psychology: Considerations for treatment selection and implementation. Dissertation, Palo Alto University. 
Parks, A.C., Schueller, S. & Tasimi, A. (in press). Increasing happiness in the general population: Empirically Supported Self-Help? To appear in I. Boniwell & S. David (Eds.), Oxford Handbook of Happiness. Oxford: Oxford University Press. (http://hiram.us/h/images/pdfs/psychology/parks-increasing-happiness.pdf).
Sin, N. L., & Lyubomirsky, S. (2009). Enhancing well-being and alleviating depressive symptoms with positive psychology interventions: A practice-friendly meta-analysis. Journal of Clinical Psychology, 65(5), 467-487.
Wood, A. M., Froh, J. J., & Geraghty, A. W. A. (2010). Gratitude and well-being: A review and theoretical integration. Clinical Psychology Review, 30(7), 890-905. 

