Triggers and clear rewards are what works for me.
Have something laid out that that doesn’t happen unless you follow your resolution, that’s your reward. Even if it’s just a picture in your mind of how your life will improve or who you’re helping.
Then have a trigger somewhere that reminds you. If you want to watch what you eat, the sound of the refrigerator opening is a great trigger. If you want to be focused on something when you sit down at your computer, make your password a trigger. Want to quit smoking, count the cigarettes in the pack every single time before having one. Something that makes you stop for a half second and think about what you’re doing next.
It’s not specific to resolutions, but for goals that take some time and applied willpower it seems to work well if you’re not too hard on yourself. Progress is more important than total conformity to whatever you’re trying to change.