After I'd written parts I, II, and III, I realized there is another common method of self-sabotage that people engage in.
#4: Changing Programs. No, I don't mean changing the program, although that's pretty common, too*. I mean changing programs (or even goals) before you've completed the first one. You start on Westside for Skinny Bastards and then switch a few weeks later to some program designed to get you to 100 consecutive pushups and then to this cool program you heard Georges St. Pierre did to get ready for his last fight only to drop it weeks later and give kettlebells a try. Oh, and work up to a 10K concurrently with all of this because runners are in shape, you know. You don't finish the whole program through once - never spending 12 weeks or 16 weeks on one program - but switch back and forth or jump around looking for the best one.
By doing so, you lose the biggest and best method of getting results - putting in work consistently. Even a not-so-good plan for your goals is going to get you better results if you stick with it than hopping around between 2-3 better plans.
This one is the most perplexing to me. Training ADD implies to the outside observer that you are more interested in a quick fix or in novelty than in consistent, hard work. That's not always fair, but if you can't stick with one thing, how do you expect to get results by doing 2, 3, or 4 of them? Unless the program you've chosen was just clearly, vastly wrong for your goals, you'll probably get more out of it by sticking to it than by switching around. We're all eager for those week 12 or week 16 numbers or results . . . but you can't get them unless you do weeks 1-11 or 1-15 first, and consecutively.
* Usually changed for the worse, too. "I'm doing Starting Strength, but I swapped out Back Squats for Smith Machines squats for 3 x 10, doing it twice a week, added 3 x 10 curls to each day, and dropped the deadlift." Flash forward three weeks - "This program suxxors, I didn't get strong at all!"