If I were you, I would begin by thinking about this novel as an excellent presentation of "The Other," which is a very important concept in postcolonialism. Basically, the idea is that we define ourselves as the colonial power as essentially good, liberated and civilised. Therefore the colonial people that we come to have power over are "othered" into the position of being bad, ignorant and uncivilised. A series of binary opposites is thus created to describe both ourselves and the way we define the colonial people in opposition to ourselves. We can see this in operation in this excellent novel with the way that slavery is presented, but at the same time we can see the way that Twain questions these easy distinctions.
Throughout this novel, we are presented with a series of characters who are seemingly very civilised and polite, who then talk about slaves as if they were animals. A prime example is when Huck disguises himself as a girl and chats with the woman in Chapter Eleven. Although she is very pleasant, kind and hospitable, she then talks about sending out her husband to hunt down Jim as if he were a dog:
"After midnight he'll likely be asleep, and they can slip around through the woods and hunt up his camp fire all the better for the dark, if he's got one."
Then, we have the classic example of Aunt Sally and Huck, who, when asked by Aunt Sally if anyone died, replies "No'm. Killed a nigger." In both of these examples, we can see the way that slaves are presented as less than human, and likewise the way in which Twain explores this process of othering. Aunt Sally is shown to be a good Christian woman, just like the other woman in Chapter 11, and yet both show themselves to lack compassion and understanding and moral superiority through the way they treat slaves.