Preventive measures

There are no 100% preventive measures, only positive factors that can reduce the risk of the development of Alzheimer's disease:

As you will probably notice by reading the formulations of the preventive measures listed below, much about Alzheimer's disease is unexplored and definitive statements about its most promoting influential factors and which preventive measures are best to be taken to reduce the risk of Alzheimer's disease can't be made.
In the current stage, it is rather so that all positive influences on the brain can be seen as preventive measures. Drugs or vaccines to prevent the development of Alzheimer's disease aren't available yet.
Straining the brain by striking (e.g. in boxing), shocking, imflammations or bleedings should generally be avoided, as it may cause the emergence of free oxygen radicals. These strains represent a negative factor for the brain
On the other hand, it can not be said that someone who had two concussions in his life or any boxer has a high risk of Alzheimer's disease.
Indicative in this context are the results of the so-called nun study . The Nun Study is an american study in which nuns of a convent from mid 70 to 100 years have participated for 10 years and had their brains examined after their deaths.
It was found that many brains showed the beginnings of plaques typical in Alzheimer's. Nut Alzheimer's disease only developed in the nuns who previously had micro-infarctions in the brain. These micro-infarctions are states in the brain where nerve cells don't receive oxygen any longer due to the blockage of blood vessels and die afterwards. Such an infarction or stroke may be promoted by increased blood pressure, inflammations or bleedings in the brain. At this point, the diet also plays a role again. If the patient's diet is unbalanced and not enough vitamins are taken, then the brain is less resilient to such an infarction. If the patient is a long-time smoker in addition, the cells may be further weakened due to the increased occurrance of oxygen radicals.
The process of Alzheimer's disease until the actual outbreak or until the date on which the disease is detected can take up to 30 years. The contributing factors to the disease during these 30 years can usually no longer be accurately determined. To determine the origin of the disease, large-scale studies, which run several decades, would therefore have to be carried out by people in different life situations. The above-mentioned nuns were selected in this context, since they had received the order to keep diaries by their employer. Conclusions about the lifestyle, nutrition and education of the nuns could be inferred from these. 
In general, it seems clear that both the physical and mental fitness are among the preventive measures.

Mental fitness

Especially in recent years the term brain jogging has become increasingly popular. So it should be the target to avoid stupefying activities, which in principle don't need any thinking processes, and to perform challenging mental tasks instead. If a repetition of certain activities can't be avoided at work, it is advised to pursue an intellectually challenging hobby to train and promote the own brain. Continuous learning is a crucial factor here. Only in this way our brain is properly used and not only minimal parts. Returning once more to the term brain jogging, there are various games and books, or even quiz shows today, which are intended to help people to learn continuously and to train the brain.

An Institute in Munich has carried out a similar study (for the nuns study, see above) in the Order of the poor. The following results emerged from the study:

Physical fitness and diet

In the area of physical fitness, movement is essential. With more exercise the blood circulation increases, and the brain is better supplied with oxygen.It never hurts to do sports or other activities outdoors.

In terms of diet, vitamin B and folic acid should be ingested. Folic acid plays an important role in cell division and is therefore a positive factor for the brain. Vitamin B lowers homocysteine levels. Homocysteine in turn may promote Alzheimer's disease. Folic acid is specifically included in green leafy vegetables but also in nuts, tomatoes and whole grain products.

Lowering cholesterol

Another negative factor, not only on the entire body, but also on the brain, is an excessively high cholesterol level. Studies have found that elevated cholesterol levels, especially LDL-cholesterol (LDL for low-density lipoprotein) is harmful to the brain and increases blood cholesterol levels in it. Too much cholesterol promotes the accumulation of amyloid-beta peptide, which in turn develops the plaques  in Alzheimer's disease. A too high proportion of LDL-cholesterol and an increased incidence of free radicals (e.g. by smoking) promotes a damaging effect on the brain. One area of research is thus devoted to modifying cholesterol-reducing drugs, so that they also affect the brain, and so the formation of plaques doesn't happen. Preventive measures to lower cholesterol levels are the reduction of fatty food (especially saturated fatty acids) and sufficient physical activity.

Diabetes, a cause of the disease?

Another disease supposed to promote Alzheimer's is diabetes. It is assumed that the insulin disorder damages the blood vessels in the brain and can thus promote Alzheimer's disease. In addition to Alzheimer's, incidences of diabetes are also increasing, which is caused by the too rich and sweet food we eat.
Secondary plant compounds (polyphenols) are therefore conducive to a healthy brain, which can be found in, inter alia, parings of fruits and vegetables instead of excessive consumption of candy. On this topic, a study has been done at the Verderbilt University Medical Center which showed that periodic consumption of fruit juice can decrease the risk of Alzheimer's disease, because the test subjects had an up to 76% slower cellular oxidation. Simple said, the brain ages slower and the cells remain resistant for a longer time.

Moderate consumption of nicotine and alcohol

Another factor is avoiding smoking and unhealthy diet. Nicotine contributes to the accumulation of pollutants in the body, which can lead to various diseases and therefore are not good for the brain, too. Nicotine further promotes the formation of oxygen free radicals in the body. These radicals can be only countered by healthy, vitamin-rich food.

In people with an increased alcohol consumption, the presence of folic acid is reduced. Apart from the reduced presence of folic acid, alcohol consumption damages the brain, anyway. It should be clear that frequent intoxications with possible memory lapses are not conducive to the health of the brain.

Normal blood pressure

Another stress factor for the human brain is hypertension. If the blood pressure is too high, it may be that the brain and the cells get damaged, too. If a raised blood pressure is existent, antihypertensive drugs should be taken in consultation with the doctor. Tests in patients suffering from hypertension, which have taken cholosterol-lowering drugs at the same time, showed that the risk of Alzheimer's has decreased in them. This could also be a cross-reference to the above-mentioned nuns study, because raised blood pressure can lead to infarctions or bleedings more easily.

Treatment of depressions

Although outsiders can't notice a direct correlation between depressions and the emergence of Alzheimer's, studies have shown that patients with depressions also had increased levels of amyloid in the cerebrospinal fluid. Again, this is also present in greater extent in Alzheimer patients. Furthermore, a disinterest in food can be existant in addition to the depressions. This could mean, that the patient has an unbalanced diet and therefore the brain's resistance is lowered. If signs of depressions occur, a doctor or psychologist should be consulted to avoid the depressions becoming chronic.