Bhagavad Gita 8.6यं यं वापि स्मरन्भावं त्यजत्यन्ते कलेवरम्।तं तमेवैति कौन्तेय सदा तद्भावभावितः ॥

The last thoughts determine the next birth. The most prominent thought of ones life occupies the mind at the time of death. The predominant idea at the time of death is what in normal life has occupied his attention most. The last thought determines the nature or character of the body to be attained next. As a man thinketh, so shall he becometh.The force of Samskaras which one has created by his previous practice is the cause of their remembrance at death. Those who have practised worship of God throughout their life can have remembrance of their tutelary deity at the time of death.The analogy of the wasp and the caterpillar (BhramaraKitaNyaya) can be applied here. The caterpillar constantly remembers the wasp and becomes eventually transformed into a wasp. Even so he who constantly remembers his tutelary deity becomes identical with that deity. Nandikesvara is an example. He constantly thought of his Lord and assumed a form eal to that of the Lord.If you constantly think of the immortal Self during your lifetime, you will entertain the thought of the Self only even at the time of death and will attain immortality. If you always think of your body and identify yourself with the perishable body you will be born again and again. If you think of you pet dog at the time of death you will be born as a dog