1. Really, don't get your hopes up on your New Year's resolution. We all know how fast these things go by the wayside.
2. If you still want to do one, pick something you were about ready to do anyway. That will give you the best odds. Arbitrarily picking something way out of left field and starting it on an arbitrary date on the calendar just leads to madness. You're ready when you're ready, not when the calendar says to be ready. If you don't have a goal you're ready to move on as it is, don't bother.
3. Pick something halfway achievable. It's not quite as satisfying as a big-time, life-changing goal, but setting your sights lower increases your odds of actually following through. 'Lose 50 pounds', for example, probably not going to happen. 'Lose 5 pounds by X date', you've got a better shot, and then once you hit that number, you can just set it again, at your own pace, and go for 10, and 15, and 20, and if you do it enough, eventually you'll get the 50 anyway.
4. Which reminds me, set little checkpoints along the way. Big, tough goals become a series of bite-sized, easily reachable goals. Have some sort of plan.
5. Tell people what your resolution is. You have more people that can remember the resolution is there that way, or at least, you have people that can hold you to it and mock you the second you look like you're failing.
6. If you do start slipping, remember that you have a resolution. If you're serious about it, you'll at least make an attempt to recover. If you just immediately go, 'Well, that's that resolution broken', you weren't serious.
7. New Year's resolutions, by definition, start on January 1st. That is tomorrow. Whatever resolution you make, at least adhere to it tomorrow. Because if you can't even do that, I can't help you.
So, hey, good luck to you, and happy 2013.