Source: A Druid's Herbal, by Ellen Evert Hopman

Tinctures are made by grinding the leaves, roots, or other plant material with a mortar and pestle (or a blender) and just barely covering them with high-quality vodka, whiskey, or grain alcohol. High quality alcohol must be used in order to extract the essential oil from the plant material. Water will not work for a tincture. Shake well once daily. After 21 days strain and store in amber (or dark) airtight containers. Keep the herbal tinctures in a cool, dry place for up to five years. As a general rule resins like dragon’s blood, frankincense and myrrh do not tincture well. The dose is generally twenty drops in a cup of herb tea or warm water four times a day. In acute or emergency situations the dose is given more frequently; in the case of labor pains, for example, it might be a dropperful every five minutes. 


From Merddyn2002:

Experience has taught me that vodka generally makes a more balanced tincture because it has roughly equal parts water and alcohol and is relatively tasteless. by using both water and alcohol for every tincture you generally get both water and alcohol soluable compounds out of the herbs. 

As a general rule it's best to tincture individual herbs, rather than blends simply because it's easier to mix and match herbal treatments if each one is concentrated separately.

I generally only keep a tincture for 2 years from the time it's completed.