As far as the issue of "how should one greet his brethren and friends" is concerned, the directive of the Shari`ah in this regard is restricted to the fact that a Muslim should greet his brethren by praying for them in the words: "Assalaam Alaikum". This is the greeting that was taught and promoted by the Prophet (pbuh) in his Sunnah. As far as shaking hands or hugging or brushing cheeks or kissing the cheeks or kissing the forehead are concerned, none of these is a part of the Islamic Shari`ah or the Sunnah of the Prophet (pbuh). All these styles of greeting friends and relatives are parts of the cultural and social norms and may vary from culture to culture. Islam has not prohibited them and it has neither promoted them by making them a part of the Shari`ah.
In my opinion, therefore, the important part in greeting our brethren is that it should be with the words: "Assalaam Alaikum". Besides this, any style of greeting that is accepted and liked in the culture may be adopted except that which may be against any of the explicit directives of the Qur'an and the Sunnah.
Soulja