-If you were sent back to the 1970s like some characters on the show, and you were 'trapped' on the island, why wouldn't you try to leave yourself a note to find when your plane crashed there in the future? How would you go about attempting to keep a note safe and in some sort of marker that you would find?
I would go to the first place I clearly remember and plant the note in a metal pipe and seal the ends, it would look like a pipe bomb. I would then cast a tall curb in cement there (in the future, I would trip over it) and write 'dig here, note in pipe' in the cement.
-What would you say in your note, 'Henry Gale is a lier, his name is Ben Linus.' might be a good start.
The most difficult thing would be; when I was done with the note and had buried, sealed and cemented it, something else would happen that I would probably feel necessary to include in said (previously buried) note.
How would one know when to stop writing and seal it up for good, I would include in the first note the location of a possible second note and in the second directions to a third. That would avoid me digging up the note repeatedly, but would lead to the situation where I would leave directions to the location of a subsequent note and then not have the time to write it and hide it properly.
Information from the note(s) would be invaluable epically to warn people so they could try to avoid their needless deaths. We have already seen that this doesn't work and the people die anyways, but I would try, just the same and may include that 'I know it won't really help but...' in the note (s)
Then there is the possibility that a second party (some guy from DHARMA or an unidentified Other) would find the note or a latter part of the series of notes, this could lead to some truly unique situations and would throw a major wrench into the cannot change the past theory.
Due to the improbabilities that this course of action would invite, and the fact that it would totally mess up the entire series, one can see why such an event was left out of the script. I however, cannot imagine that it wasn't thought of, discusses and dismissed. It it an interesting thing to think about.