Coffee dehydrates and thus makes you more stiff. Greens and juices make you more flexible. Therefore give up coffee and drink juices, correct? On the other hand, research has shown that drinking at least 3 cups of coffee per day may be linked to the prevention of alzheimers. Additional very large scale and significant studies have shown that drinking up to 4 cups a day significantly lowers your risk of developing diabetes and is not linked in any way to increased rates of heart disease or cancer. So which do you prefer - an elevated risk of alzheimers or slightly reduced flexibility? As coffee is certainly a dehydrating diuretic, the recommendation is that you drink water with your coffee as is practiced in many serious coffee drinking countries. In the middle east for example, where thick ultra strong Turkish coffee is the norm, a glass of water is always offered to accompany your caffeine of choice.
Follow these links if you want more detail:
Coffee & Diabetes, Cancer, Heart Disease
Coffee & Alzheimers
Pattabhi Jois, the founder of Ashtanga Yoga allegedly always had a cup first thing in the morning before his legendary practice, as did one of my own teacher's gurus, Swami Nada Brahmananda. Although a coffee addiction is of course to be avoided, my own personal opinion is to not be fanatical about it. If you enjoy a cup then go ahead and don't feel guilty up to about four cups a day because current research shows that this is good for you. It may reduce your flexbility very marginally but unless you are entering the Yoga Olympics, this is a very balanced trade off. If you aren't going for gold, then you would be advised to go ahead and have that brain stimulating cup of coffee first thing in the morning with a glass of rehydrating water. Then later in the day have your flexiblity enhancing juice and you will be balancing the best of both worlds!
PS This is not medical advice so if you have any doubts please DO consult your GP!