What are the main things I should consider before deciding whether or not a juice cleans is something I should do?
The most important thing would be determine what you hope to get out of the experience?
There is a lot, (A LOT!) of woo (pseudoscience) behind fasting for health; not so much evidence to back it up. A lot of this has to do with “Detoxing” which itself is meaningless(the body already has sufficient methods for detoxification – liver kidneys). If this is your sole purpose then I’d encourage you to check out this article at sciencebasedmedicine.org (no point for me to re-invent the wheel).
The idea that your previous lifestyle will confer to specific symptoms experienced during a fast is pseudoscience (not supported by scientific evidence) and instead may be easily explained common phenomenon when the body is devoid of nourishment.
Now, I don’t want or mean to be overly excessive in my skepticism (and likely cynicism). In some capacity, intermittent fasting has somewhat lost its obscurity in scientific literature. Both animal models and even some human trials have produced some exciting preliminary data. Overall, it exhibits a strong theoretical potential (but little more than that); As often with a niche topic, enthusiasm often exceeds the actual evidence.
The majority of research (albeit still small) deals with fasting (i.e. complete avoidance of food) as opposed to modified fasting (i.e. hypocaloric intake/juice fasting). Moreover, intermittent fasting is often purposed as a lifestyle approach (for weight loss), one which compliance is exceedingly difficult (attrition high).
If it helps you kick start a lifestyle change then that is awesome. But please be responsible (stay hydrated, do not attempt while on medication, sick or if you have a disorder of the metabolism, etc.,).
Nonetheless, I would heed what /u/BackWithAVengance posted.
If you or anyone is interested the following are the best sources on the current evidence on fasting:
Mattson, M. P., Allison, D. B., Fontana, L., Harvie, M., Longo, V. D., Malaisse, W. J., ... & Panda, S. 2014. Meal frequency and timing in health and disease. Proceedings of the National Academy of Sciences, 111(47), 16647-16653. http://www.dur-a-avaler.com/wp-content/uploads/2015/01/PNAS-2014-Mattson-16647-53.pdf
Longo, V. D., & Mattson, M. P. 2014. Fasting: molecular mechanisms and clinical applications. Cell metabolism, 19(2), 181-192. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/