The great scholar Ibn al-Qayyim mentioned the numerous benefits of using the siwak (miswak) in his Prophetic Medicine, and said that it was a commonly-used and recommended act by the Messenger (sallallaahu alayhi sasallam).
"If it were not that I would make it difficult for my community, I would order them to use the siwak before every time of prayer." (Bukhari, Jumu'a, 8).
"It is a means of purifying the mouth, pleasing to the Lord." (Bukhari, Sawm, 27).
Ibn al-Qayyim said that the best kind of siwak was made from the wood of the arak (Salvadora persica) tree and similar woods, and should not be made from an unknown tree, as it may be poisonous. He also said that the best way to use the siwak was moistened with rose water.
The great scholar said that the siwak should be used in moderation, and mentioned some of its benefits as:
Ibn al-Qayyim recommended the use of the siwak at all times, at times of prayer and for ablution, on awakening from sleep, when the odor of the mouth had changed, and for a person fasting or breaking the fast.
What Is the siwak?
The bark of the siwak is light brown and its inner surfaces are white. It has a warm and pungent taste.
The stem bark of the salvadora persica tree, the tree commonly used for siwak, contains chlorides, fluoride, silica, sulfur, Vitamin C and small quantities of tannins, saponins, flavonoids and sterols.
Health Properties of the Siwak
A study carried out in Saudi Arabia in 2003 compared the use of miswak with ordinary toothbrushing. The study found:
"It is concluded that the miswak is more effective than toothbrushing for reducing plaque and gingivitis, when preceded by professional instruction in its correct application. The miswak appeared to be more effective than toothbrushing for removing plaque from the embrasures, thus enhancing interproximal health."
Another study showed that the miswak had comparable effects on plague control, when compared to chlorhexidine gluconate (CHX), one of the best-proven anti-plaque agents. 
Tips on Using the Siwak