Ravana abducted Goddess Sita with evil intentions to marry her. But he didn't step forward to do so. What protected Goddess Sita from Ravana for more than six months (as far as I know) in Ravana's palace?
Ravana was not able to force himself on Sita due to a curse he had received years earlier, as described in this excerpt from the Uttara Kanda of the Ramayana. Ravana and his army were once camped on mount Kailash, preparing to attack Devaloka (the kingdom of the gods), when he saw the Apsara (nymph) Rambha and was enchanted by her beauty. He propositioned her, but she refused his advances, saying that she was like Ravana's daughter-in-law, as she was on her way to see Nalakuvara, son of Ravana's brother Kubera the god of wealth. Ravana paid no heed to this, saying that Rambha was an Apsara, and Apsaras are never wedded to just one husband. He then ravished Rambha. When Nalakuvara heard what had happened, he put the following curse on Ravana:
Oh fair one, since thou hast been ravished despite thy unwillingness he shall never be able to get by an unwilling damsel. Whenever he shall, stricken by lust, ravish a reluctant damsel, his head shall be sundered into seven pieces.
So Ravana couldn't take liberties with Sita against her will. He had to get Sita to marry him voluntarily, although he did use threats to try to convince her. When he first brings her to Lanka, he says this:
Oh, angry lady Maithili, listen to my words. A period of twelve months is given to you. And oh, smiley smiler, if you do not come nigh of me within that period, then the cooks will slice you to pieces for the purpose of a morning meal.
And when Hanuman came to Lanka, he hears Ravana telling Sita that he would keep his word and give her two more months before killing her:
O Seetha with best complexion! Whatever limit has been made by me to you, those two months are protectable to me. Thereafter ascend my bed. Above two months you not desiring me as husband will be killed in my kitchen for my breakfast.
(On a side note, it's probably as good a time as any to mention that Vishnu says in the Skanda Purana that Sita was never actually kidnapped, as I discuss in this answer. If that's the case, then the real answer would be "by not going to the palace at all"!)
Ravana told the reason for not forcing Sita to Mahaparsva. The reason is the fear of the curse received by him from Brahma that the his(Ravana's) head will break into hundred pieces if he tries to force a woman for pleasure. This can be clarified from the statements of Ravana in the following passage from Valmiki Ramayana.
Appreciating the words spoken thus by Mahaparsva, King Ravana spoke the following words:
"O, Mahaparsva! Know a little secret about me. I shall tell you an incident, which occurred to me long ago. Once I saw a celestial nymph, Punjikasthala(by name) flashing like a flame, concealing herself in the sky and proceeding towards the abode of Brahma. She was made unclothed by me and was enjoyed forcibly. Thereafter, appearing like a crumpled lotus, she went to the abode of Brahma. I think that the matter was made known to the high soled Brahama and then the enraged Brahma spoke to me the following words: "From today onwards, if you revel with any other woman forcibly, your head then undoubtedly will break asunder into a hundred pieces". Fearing the curse given in this manner by Brahama, I am not violently making Sita the daughter of Videha, to mount on my beautiful bed forcibly."
I will try to be logical rather than being authentic as I cannot lay my hands on any citations in support of my answer.
Ravana was never looking forward to marry Sita. The whole problem started after the proposal of marriage by Ravana's sister Shurpanakha was rejected by Rama & Lakshmana in a cruel manner. Ravana primarily came out to avenge this from Rama & Lakshmana. But after he saw the duo with Sita he was attracted to her. He decided to take her to Lanka and marry her.
After reaching Lanka his first wife Mandodari scolded him on his shameful act. He tried to argue that he did this to revenge his sister's insult and hurting by the brothers. So he wants to avenge this insult by marrying her. Mandodari advised him that if he marries Sita without her consent it would be wrong and unjust (Adharma). So he should first convince Sita and then marry after taking her consent. As per the Ramayana, Mandodari always accompanied Ravana to 'Ashok Vatika' to ensure that he is not using any force. Since Sita never accepted Ravana's proposal he did not even touch her as he was a genuine brahmin and not a rapist.