Apparently, it is extremely difficult. I can't count how often I see people going "too" the store "too" get "two" bottles of whatever. I also see people who like something, and their friend likes it "to".
The word "to" is a preposition. It shows direction. If you are moving in any direction, then you are moving to something, and away from something else. Remember it this way: "to" and "from" each have only one letter "o".
The word "too" is an additive. You have something, then you add something else; as in, "having you cake and eating it too." Remember it this way: if you are adding one thing to another thing, then you need to add one "o" to the other "o".
The word "two" doesn't seem to get mixed up as much. Everyone seems to remember that the number is the one with the "w" in it.