If you want to drink something besides water, consider green tea.
A European study now confirms that the green tea improves a crucial test of circulation. Its called flow mediated dilation (FMD). Researchers gave healthy volunteers 6 grams of green tea, hot water, or 125 mg of caffeine (the amount contained in the 6 grams of tea). The tea did not have any measurable effect on certain markers of vascular inflammation. However, it significantly improved FMD, as compared to the caffeine and the hot water.
FMD is a marker of health of endothelial function. Endothelial cells are the critical inner lining cells of your arteries, whether in your peripheral arteries or coronaries. If they are not working right, your arteries will be stiff and inflexible. Better FMD means healthier endothelial cells and vice versa. Tea, both black and green, is proving a great way to preserve and protect these crucial cells.
Ref: European Journal of Cardiovascular Prevention & Rehabilitation, 15(3):300-305, June 2008.