One big reason why we have trouble getting things done is an inability to concentrate on what we're doing. Either because we're being interrupted (by kids, pets, noises outside, coworkers) or because we are interrupting ourselves (checking e-mail, refreshing that message board, getting the latest scores).
The real problem is that we're spending a lot of time not actually doing what we're saying we're doing, and thinking that means that we can never get it done. Instead, we should learn to concentrate on what we're doing right now: to be fully in the moment. This is a principle that can be applied to every aspect of life. Instead of sitting around at work thinking about what you're going to do when you get home, or even worse, sitting around at home thinking about work, take those times and concentrate your energies. At work, do your work. I was surprised when I applied this and found myself running out of work to do halfway through the day, because I was suddenly so efficient. No more late nights, trying to get something done that I could have finished that afternoon. And that freed me to be more fully at home when I left work. I could think about what I wanted to do in my free time during that time, and not worry about a work problem.
We make a lot of excuses for why we can't get things done, and some of them are just ridiculous. If there's a noise outside, an adult human being with no diagnosed mental disorder should be able to ignore it and work. Yes, even the sound of jackhammers, and the only exception I will make is for anybody who has to listen for something fairly quiet under that sound. I worked for some time in an office next to a freight train track, and apart from it being impossible to hear each other talk when a train was going by, everybody learned to work with it.
The big excuse I hear is that there are constant interruptions from kids and coworkers. Kids and coworkers need limits set for them, and you need to learn how to set those limits. You owe a duty to both of them, either to care for them and supervise them, or to respond to them when the problem is urgent. But getting your work done means learning how to delegate responsibility, and perform triage on emergencies. It also means learning how to say, "I'm working right now; I'll call you when I have time to talk."
Like outside interruptions, ones you create for yourself can seem really urgent. You sit down to work on your novel, and the first thing you do is check your e-mail, just to be sure there's nothing critical there. An hour later, you remember you wanted to be writing, but now the time you had for it is eaten away and nothing gets done.
To deal with internal interruptions, you need to know what they are. For me, it's catching up on the various web sites I visit regularly. I often come home with a specific task in mind (tonight I will do the laundry) and the next thing I know, I've frittered away hours on stupid things, and the task goes undone. To deal with this you have to ration the interruptions. Trade fifteen minutes of engineering calculations for fifteen minutes of blog-reading, for example. Or take the laundry and homework to the laundromat, and afterward give yourself an hour of reading that great book about plant propagation. Give yourself a time you must spend doing the actual work you need to get done, and also a time you can spend doing the fun thing you want to do (or the thing that wastes your time). When the time is up, the activity is up.
Learn which tools are good and which ones are more distracting. For me, an RSS reader helps keeps the blogalanche under control. For some people, having a little desktop widget that shows the latest headlines is just too damned distracting. This is a very personal thing. You learn what works and what doesn't by observing yourself, and noting what you were doing when you got distracted.
Again, learning to concentrate and work on what you're doing without day-dreaming takes work. But the only guarantee there is in life is that if you keep doing nothing, you will get nothing done. By learning to concentrate your work, you can take advantage of concentrated doses of time, and get more of the things you want to get done done. The biggest (and worst) excuse is, "I can't get enough work done in fifteen minutes." But you can get some work done, and that's more than you were getting done before.
The key is to learn to turn on the intensity quickly, to be able to focus immediately on the task and pick up from where you were. And then, of course, to be able to turn off the intensity just as immediately. This is not easy. It does not happen the first time. It takes practice. But it is possible for anybody to learn this with time.
NOTE: No, I do not believe your self-diagnosed ADD is a reason why you might not be able to do this. If you have doctor-diagnosed ADD, you are likely taking medications that make it possible for you to concentrate, and you don't need this pep talk. If you have diagnosed yourself with ADD, you need to either see a doctor about it or admit that you're just out of practice at concentrating and get with the program. If you want to get things done, there's no excuse for letting brain chemistry hold you up, and there's no excuse for pretending brain chemistry is what is standing in your way.