Relaxation, meditation, mindfulness, hypnosis, imagery and other associated methods form a complex, loosely interlinked field.  The "Four aspects of helpful inner focus" model, that I've put together to help me make more sense of this territory, looks like this: 

For a downloadable copy of this diagram click here.  There is a fair amount of data supporting most of these methods.  To give some examples:

1.)  "Reducing negative states" - Manzoni et al's meta-analysis of relaxation methods for anxiety is clear that these approaches show "consistent and significant efficacy in reducing anxiety".  Norton et al's meta-analysis of CBT for anxiety disorders concludes that "tentative evidence suggested equal effects of CBT when compared with relaxation-only treatments."  Stetter et al's Autogenic Training (AT) meta-analysis and Krampen's study of AT and reduction in depressive relapse illustrate the value of this approach.

2.)  "Nourishing positive states" - (compassion & safety) Leary et al's studies illustrate aspects of self-compassion research, Fredrickson et al look at compassion for others, while Omri et al discuss promoting attachment/safety.  (enhancing performance)  Implementation intentions (with their associated use of imagery) is probably most relevant here (see Gollwitzer's meta-analysis), but there are other uses such as desensitisation training using imagery, and even sports performance improvement (Ollson et al).  (self healing/transformation)  Use for physical symptoms (e.g. Gonsalkorale et al for IBS) including cortical reorganization (MacIver et al).

3.)  "Encouraging mindfulness" - (mindfulness training)  Some encouraging stuff here - see, for example, last year's Kuyken et al study.  (flow & engagement) See Salanova et al as example.  (coming to our senses)  Less data here but see, for example Kabat-Zinn's book entitled "Coming to our senses".

4.)  "Exploring and processing" - (memory processing)  So much here, such as PTSD work (e.g. Bryant et al).  (dialogue work)  Used both for dialogue between internal ‘voices' and between client and important others in their life (Arntz's work as example here e.g. Giesen-Bloo et al).  (forms of focusing)  Less strong data here, but the Hendricks review is helpful.