Mantra is a combination of sounds done in Sanskrit language, that have a spiritual meaning. By voicing these sounds (preferably loud and clear, but they can also only be voiced in your head) we can tune into the energy of a certain planet. To have any effect, you should do this regularly for a defined time.
Sanskrit is the language of the Vedas, the Indian body of knowledge which predates religion and is said to have been cognized by ancient enlightened beings. It is revered as a pure language because there is essentially no difference between a particular word and the object it describes. This is called « name and form. » These prayers of a sort invoke the grace of a planetary energy to lessen its negative effects as revealed in the horoscope.
Regarding the procedure of chanting, one may be sitting, standing, or walking while practicing, but the best effects are gained when the chanting is done softly and gently. Although the instruction of chanting does not include dwelling on the meaning of the mantra, the process should be a conscious one. During the practice one may notice his attention easily focusing on the vibration of the sound within the body. A very peaceful and relaxing state will result when this is done properly. The chanting should be smooth and repeated somewhat in a monotone to fully appreciate the vibrations, but the rate of speed may be as quick as desired.It is primarily the vibration of the sounds on the human nervous system which will cause the subtle positive effects on the person and neutralize the harmful energy indicated by an afflicted planet. 

A string of 108 beads called mala is used for the counting, and one bead, known as the Guru bead, is slightly protruded from the others so one knows when the mantra has been chanted 108 times. At the conclusion of one mala the person either stops or turns back, so the Guru bead is never crossed.
During the practice, one bead is kept between the thumb and middle finger to keep track of the number of repetitions. The most desirable beads, according to many Hindus, are rudrakshas, which are found In India or Nepal and are considered somewhat sacred. The next best are tulsi beads.

 

MALA 1
Mala made by Nila with prescribed gems and rudrakshas

 

MALA 2

BEEJ (SEED) MANTRA

Beej Mantras also known as Vedic Seed Mantras are the core mantras or sounds endowed with great spiritual powers, They are often called the audible seed version of all the Deity majorly in Hinduism. Beejas form part of several mantra compositions and hence they are like the batteries of mantras. It is believed that when chanted with concentration Beej mantras can fulfil the desires of the devotees and act like a protective shield surrounding them and protecting from all dangers and enemies.

OM ॐ is a sacred sound and a spiritual symbol in Indian religions. It signifies the essence of the ultimate reality, consciousness or Atman. It is related to the Sun. The upanishads say that the Sun chants OM as it moves in the sky. Om is the sound of the essential cosmic vibration transmitted through the sun. Om is good for all planets.All mantras shoud begin and end with OM.

SHRIM (shreem) relates to Goddess Lakshmi, Moon and Venus
HRIM (hreem) is the beej mantra of the divine Mother, it is the feminine form of OM. It also relates to Mars and Venus
KLIM (kleem) relates to goddess Kali. It is good for Saturn and Rahu.
KRIM (kreem) is for Krishna, venus and the Moon
GAM (like gum) for Ganapati and Jupiter.
AIM for Saraswati and Mercury.

Specific mantras (with beej) for the planets

SUN
OM SUM (soom) SURYAYA NAMAHA

MOON
OM SOM (som) SOMAYA NAMAHA
OM CHAM CHANDRAYA NAMAHA

MARS
OM KUM (koom) KUJAYA NAMAHA
OM MAM MANGALAYA NAMAHA

MERCURY
OM BUM (boom) BUDHAYA NAMAHA

JUPITER
OM BRIM BRIHASPATAYE NAMAHA
OM GUM (goom) GURAVE NAMAHA

VENUS
OM SHUM (shoom) SHUKRAYA NAMAHA

SATURN
OM SHAM SHANAYE/ SHANAISHCHARAYA NAMAHA
OM NAMAH SHIVAYA
OM SHANTI SHANTI SHANTIHI

RAHU
OM RAM RAHAVE NAMAHA

KETU
OM KEM KETAVE NAMAHA