So much of training for, and competing in, any sort of endurance sports (and probably other forms of working out too, like the gym) is mental. I have found that one nearly foolproof way of ensuring that I actually do the workouts I need to do is to come home from work and immediately (well, immediately after letting the puppy out) put on swimsuit or running/cycling clothes. If I do that, I am at least 90% sure I'm actually going to do the workout. I may have thoughts of skipping it after I get dressed for it, but those thoughts will bring shame upon me and I probably won't act on them. The thought of the defeat inherent in the act of taking off clean, non-sweaty workout clothes one piece at a time and folding them and putting them away, and then returning to whatever unproductive or less-important-than-the-Boston-Marathon thing has distracted me, is too much for me. I think I would not be able to stand myself if I were to do that. In fact, even though I said above that I will do the workout 90% of the time if I dress for it before doing anything else, I actually can't think of a single time that I have dressed for a workout and then CHOSEN not to do it. There have been a couple of times when injury, illness, or urgent circumstance have chosen for me, but I can't think of a single time when I blew off a workout for no good reason after getting ready for it. So one mental game to play with yourself in order to get yourself out the door, is to put on the clothes as soon as you come in the door from work. (Or even AT work, before leaving; that would probably be even better!) Do not, under any circumstances, sit down at the computer or, even worse, on the couch with a book and tell yourself it's only for a few minutes. It won't be. Once you sit down you'll be thinking about how tired you really are and how, gosh, you worked so hard today and you really deserve a little nap. Then you'll wake up and the sun will be down and the air will be chilled and you will know you're not working out at all and since you're such a lazy, fat slob you may as well go ahead and eat that Rocky Road ice cream in the freezer. In for a dime, in for a dollar! No, better all around to just put on the clothes and go do the workout and then feel great afterwards and eat that ice cream because you earned it and not as a consolation for being fat and lazy.
Another mental thing that helps me tremendously is to keep records of, not only exercise, but also calorie intake. If you are compulsive like me, what you should do is this: go out and get a free computer program that tracks both calorie intake and calories burned, and record everything. The need to get those two numbers in a healthy proportion (to me, that is a 3:2 ratio of calories consumed:calories burned, so if I burn 2000 calories, I should eat about 3000) will become a radical driving force and you will not be able to rest if you ate and didn't exercise. (Disclaimer: I use this formula only because it worked for me prior to the San Diego Marathon, and let me lose weight and not feel like I was dying of starvation the whole time. It is not by any means a scientific formula, or developed by people who actually know what they're talking about, like dieticians, or anything like that.)
The last thing that really helps, for those days when you have dressed for the workout and even managed to drag yourself to the site of the workout (the pool, or the park, or standing in your driveway holding the bike) and you just plain do not want to do it, is to tell yourself that you only have to do a minimal distance (a mile of running, 2 miles of biking, 800 yards in the pool) and then you can reevaluate and quit if you truly do feel lousy or are hating the workout. 9 times out of 10, knowing you have the opportunity to bail if you want to will get you moving, and when you hit your reevaluation point, most of the time you will just reason that you're already far enough in that you might as well just keep going.
I hate to work out, as everyone knows, and these three strategies have done a pretty good job of forcing me out the door for the last 5 years.