Rule 1: You may end a sentence with a preposition. Just do not use extra prepositions when the meaning is clear without them.
Examples
That is something I cannot agree with.
How many of you can I count on?
Where did he go (to)?
Where did you get this (at)?
I will go later (on).
Take your shoes off (of) the bed.
You may not look out (of) the window.
Cut it (up) into small pieces.
Rule 2: Use "on" with expressions that indicate the time of an occurrence.
Examples
He was born on December 23.
We will arrive on the fourth.
Rule 3: "Of" should never be used in place of "have."
Correct: I should have done it.
Incorrect: I should of done it.
Rule 4: "Between" refers to two. "Among" is used for three or more.
Examples
Divide the candy between the two of you.
Divide the candy among the three of you.
Rule 5: "Into" implies entrance, "in" does not.
Examples
Sally walked into the house.
Sally was waiting in the house.
Cut the pie into six slices. (The knife enters the pie.)
"Bad language is viral; it's in the atmosphere, and we all pick it up." - Robert Hartwell Fiske