Shanidev is the most feared deity in Hindu Religion. He plays the role of a judge in human lives and depending on our Karmas reward or punish us. According to Vedic Astrology Shanaiswar is representation of the Planet Saturn and is a very slow moving Planet. Saturn is generally known to affect one adversely on occasions when he occupies certain positions in one's horoscope like the Saade Sati (7 1/2 years) ,Ashtama Shanaiswara (Saturn in 8th house - 2 1/2 years) etc. Chanting the Shani Mantra reduces the malefic effects of Saturn in Horoscope. Given below are some Mantras of Shani dev.
"Om Hlim Sham Shanaye Namah"
"Om praam preem praum sah shanayishraya namah"
"Om sham shanayscharaya namaha"
"Aum Aing Hring Shring Shung Shanaishcharaye Namah Aum"
Shani Gayatri Mantra
"Om Sanaischaraya vidhmahe
tanno manda prachodayat"
Shani Dhyan Mantra
Nilanjana samabhasam raviputram yamagrajam
chaya martanda sambhutam tam namami shaishcharam
I bow to Lord Shani, who is black in colour and son of Sun and born to Chaya and brother of Yama , who moves very slowly.
Om Shanno Devirabhishtaya Aapo Bhavantu Peetaye Shanyorbhisravantumah Shanaishcharaaya Namah
Read more on Shani and remedies for shani affliction