Hope you're ready for some reading material!
No matter what you do, time will correct itself. Maybe some of the Losties went back in time to avoid the crash, or maybe they relived certain defining moments. No matter what, though, they still ended up on the island.
Example 1: In "Further Instructions", Locke is at a commune that is in the Mary Jane business. Now Locke is mysterious, but I can't believe that he would be into drug trafficking. Maybe this is the alternate timeline if he had never chased after his mother in the toy store parking lot. His dad would've still found him and soon paralyze him, and he would still end up in Australia. This is why I think that the woman he is talking to on the phone in "Walkabout" and the woman he meets in "Orientation" are the same Helen. If John and Helen never met, she would never know happiness with John and end up being a phone sex operator(I'm guessing that's what she was in that episode).
Example 2: In "Stranger In A Strange Land", Jack is in Phucket and has a relationship with Achara. Maybe this is his alternate timeline. This is what would happen if he chose to save Adam Rutherford instead of Sarah. Jack may be annoying, but he's proven that he won't cheat on Sarah(He pulled away when Gabriela kissed him in "The Hunting Party"...and she was HOT!). So maybe he never knew or married Sarah. But his dad still died in Australia, and they both ended up on 815.
Example 3: In "Every Man For Himself", Sawyer is in prison. This would be his alternate timeline if he went through with conning Jessica and David instead of walking away when he sees their son. He cons them, but gets caught and ends up in the slammer. He still ends up in Australia and gets on 815.
There are other scenarios, but I'll let you ponder these first.
It's out there. Tell me what you think.