The key is to eliminate the source of the friction. You can achieve this in several ways:
On the feet: wear well-fitted (not too tight or too loose), comfortable footwear and clean, dry socks. If your shoes fit badly or are too stiff (high heels for example), your feet are more at risk of blistering. Moist skin tends to blister more easily, so socks that manage moisture or frequent sock changes can be helpful. If you are prone to excessive sweating, the use of talcum powder might also help. If you are exercising or practicing sport, wear specially designed sports socks that will help reduce foot sweat or friction. In addition, hiking or walking boots need to be adequately broken in before embarking on a long trek.
Applying preventive solutions such as tape, padding or moleskin to trouble spots (based on personal experience) can help prevent blisters from appearing. You can also use friction-management patches which are applied to the inside of shoes. These will remain in place longer, throughout many changes of socks or insoles.
On the hands: Depending on what is your favorite activity, you are more or less at risk of developing hand blisters. If you are using tools or carrying out manual work, wearing gloves should help prevent blisters. The same goes if you are playing a sport where you are holding a bat, racket, club etc. For some other sports such as gymnastics, weightlifting or rowing, applying tape on the hand is an effective way to prevent blister occurrence. Additionally, the use of talcum powder in order to reduce friction could prove beneficiary in combination with gloves or as a stand-alone option. Keep in mind, however, that talcum powder also absorbs moisture and as such is not a good option for prolonged activities.