Ramban (כה:לב) says that it is reasonable to assume that the law stating that the first born inherit the double portion was not yet instituted at the time עשו sold his birthright. The main aspect of the first born's inheritance was the respect and honor of the father. All prestige and authority that the father had, the son received. Eisav was not concerned with the monetary aspect of the inheritance because, according to the Ibn Ezra, Yitzchak was poor at this time, having used up all the money left to him by Avraham (Rambam strongly disagrees with this point of the Ibn Ezra). Many commentators agree that Eisav did not want or need the spiritual burden of inheriting Yitzchak's respect and authority so he gladly sold the rights to Yaakov.
For what did Eisav sell his birthright? According to the Torah it seems to be lentil soup. This appears strange, for who would sell anything, even if you did not see it important, for lentil soup? The Rashbam offers an explanation. He says that the lentil soup served merely as a sign that the sale had occurred. Yaakov actually gave his brother money in return for the birthright. Eisav gladly sold the birthright, for he saw no good in it and took the soup to ratify the sale.
Sforno agrees with the Rashbam that the lentil soup served only to ratify the sale. He provides us a sequence of events: Eisav arrived home, tired from hunting. He saw the soup and asked Yaakov for it and Yaakov gave it to him. Yaakov then suggested to Eisav that if you are so preoccupied with your work, you certainly will not be able to fulfill the responsibilities required by the birth right, that is to serve Hashem. Eisav agreed with Yaakov that he would die of exhaustion if he would hunt and then have to carry out the responsibilities of the birthright. He then volunteered to give up the birthright; Yaakov made sure that money exchanged hands and that Eisav swore about it so that he could not complain later.