I have read a few books on Neurolinguistic Programming (NLP) and, it seems that a corner stone of NLP is strong inner visualisation skills.
Is there any research that looks at the correlation between visualisation ability and the effectiveness of NLP?
Are there any scientifically proven techniques for measuring and improving visualisation skills?
NLP (Neurolinguistic Programming) is a pseudoscience at worst, and not a science, but "an approach to communication, personal development, and psychotherapy" at best. There is little scientific research, and no reliable scientific findings, supporting the effectiveness of NLP, and the evidence is probably best interpreted as speaking against the effectiveness and veracity of NLP 1, 2, 3.
In non-scientific venues, there are indeed sometimes claims that NLP can help with visualisation as understood by NLP, e.g. here. In contrast, a recent experimental finding that is actually scientific disconfirmed NLP claims about the nature of visualization as conceptualized by NLP.
It should be noted that there seems to be little overlap between visualization - if assumed to refer to the same concept as mental imagery - as employed by vision researchers and cognitive scientists studying vision, and the NLP concept of visualization. The latter instead vaguely resembles some concepts in cognitive linguistics (e.g. research into metaphors such as those by George Lakoff).
I also want to emphasize that Neurolinguistic Programming is not neurolinguistics, which is a legitimate subfield of cognitive science: the study of how the brain implements language. In contrast, NLP can most likely justifiably called quackery.
