The superstitions mentioned above are specifically from chapters 6-7. I thought it would be neat to mention a few more from later in the selection you suggested. Therefore, the ones I am explaining begin near the end of chapter seven and go through chapter eight. The superstitions in this section are no less interesting!
A later interesting superstition in chapter seven involves "being engaged." In school, Tom asks Becky to become engaged to him. Becky is confused. Tom explains the superstition: the girl and boy kiss, the girl and boy promise to love each other forever, and the girl and boy promise to marry each other when they grown up. This equals engagement according to this superstition!
In chapter 8, there is a really interesting superstition regarding marbles. You see, Tom is getting his belongings together (in order to become a pirate) and digs up a box containing only a single marble. Tom stares at the box blankly, and in disbelief for a while. Why? Because Tom believes that if you say a certain chant and bury a marble for two weeks, the rest of your lost marbles will magically join it. This superstition obviously didn't work.
Yet another interesting superstition immediately follows the one about the marbles. It is about witches this time. Tom blames the failed marble incantation on the interference with witches. It is a further superstition that Tom "knows" he can never work against a witch in any way; therefore, he gives his marble up for lost. (Except the one in the box, which he mistakenly tosses aside and then finds.)
In chapter 6, Huckleberry asserts that dead cats cure warts. However, Tom insists that spunk-water (the rain-water collected in an old rotten stump) is better at curing warts. He even proves it by telling the story of a man who did it with success. Huckleberry argues that it has to be done a certain way, at midnight, and gives the exact steps that must be taken as well as the magic words to say. Tom also explains how to remove warts with a bean and Huck returns by explaining how to use the dead cat. Huck also explains how witches cast spells - by saying the Lord's prayer backwards. He insists that one cast a spell on his Pap once.
In chapter 8, Tom is disappointed that one of his superstitions had failed. His marble, which he had recited incantations over, hadn't produced his other lost marbles. So instead, he speaks magic words to get a little black bug to come out, who, by not telling him anything, indicates that a witch made his spell not work. Then he tries another superstition by trying to get the marble to locate its lost "brother". His third try is successful.
In Chapter 6 (page 2 in the enotes copy of text) there are at least three superstitions about getting rid of warts. One is with dead cats, graveyard and a chant. You must go to the cemetery after someone "wicked" has been buried and when the devil comes to get them, "you heave your cat after 'em and say, ‘Devil follow corpse, cat follow devil, warts follow cat, I'm done with ye!'".
The second deals with plunging your hand in a stump filled with "spunk water" and chant “Barley-corn, barley-corn, injun-meal shorts, Spunk-water, spunk-water, swaller these warts, “and then walk away quick with your eyes shut and then walk home without speaking to anybody.
Third wart removal technique involves a bean. “You take and split the bean, and cut the wart so as to get some blood... bury it 'bout midnight at the crossroads in the dark of the moon”.
Another superstition is "witching" someone. Huck says that someone"witches"you when they stare and "mumble". The mumbling is "them saying the Lord's Prayer backward."