In order to hit 100% game completion you don't actually have to quite everything in the game (for example, it's not necessary to do all the Stunt Jumps, Under the Bridge or Knife Flights): the 100% total is made up of a series of items found on the R* Social Club checklist. You can see the progress through the in-game menus but the Social Club page gives you considerably more information.
The 100% is made up of the 69 main story missions as well as 42 particular Hobbies and Pastimes (such as the races, flight school, etc.), 20 particular Strangers and Freaks missions, any 14 random events (the ones that pop up as you're just driving around) and 16 particular miscellaneous things you can do in the game (such as holding up a store).
It's worth noting that in order to get 100% you will need to collect all of the spaceship parts and letter scraps (though it's not necessary to do the submarine parts or the nuclear waste).
To track it on their site, once you have an R* Social Club account head to this page: http://socialclub.rockstargames.com/games/gtav/career/checkl...
Hope this helps those going for 100%. If you have any comments, or additional advice drop a comment.