I haven't found any one hack that has worked for me, more like a culmination of many small things done consistently that become habits and together can make a fit person. I think there are two big mistakes people often make with resolutions like those at new years:
1. They pick something that's too difficult - It's really easy to set a huge goal, like going low carb, running a marathon something else without really understanding the lifestyle changes that need to come along with that. It's better to do small things and make them habit.
2. They set outcome goals instead of process goals - People want to have an outcome like "I want to look sexy like that person" or "I want to lose 100 lbs". It's much better to set process goals. Like "I want to make it a habit every day to get 5000 steps on my fitness tracker instead of the 2000 I have been getting".
That being said, here are the things I could think of that are pretty easy to do but collectively will lead to a better lifestyle and healthier habits. Don't try to do all of these at once. Pick one and focus on it for a while until some progress is made, then pick another one.
Walk more - Get a fitness tracker and keep track of how much you are walking. Getting frustrated with something at work? Take a walk. Feeling sluggish? Take a walk instead of having a snack / coffee (ok who am I kidding, have the coffee anyway). Walking is great because it is activity that is almost all fat burn and doesn't activate hormones that tell your body to eat like intense exercise.
Workout for lunch (or dinner) - Feel like taking a break and sitting down to a big meal? Go workout while you are doing it. I'll often go ride my bike at lunch time and eat some sport snacks on the ride, and forgo a big lunch. But make sure to still eat something!
Track calories (but not all the time) - Just for a couple weeks track how many calories you are eating, and the macronutrient composition (fat, protein, carbs). It's pretty surprising the trends you will notice and adjustments you can make.
Eat less sugar - This one is kind of obvious. You don't have to cut it out entirely, but once you start paying attention it is amazing the kinds of things that have sugar added to them. Bread, cereal, yogurt, drinks etc. Reducing blood sugar fluctuation makes self control a lot better, and makes all of these things easier.
Drink less alcohol - I love alcohol, but it's basically pure carbs with no nutrients or fiber. Plus it decreases your body's ability to heal itself. Just cut out a few drinks when you can.
Eat more fiber - More fiber means more satiation. So your meal will go a much longer way. It also means that food is slower to digest, so there is less of a spike in blood sugar. It doesn't have to be salads all day long. In my experience comparing a product like bread, sugar added and fiber content are inversely proportional; So it's kind of a win win.
Weigh yourself every day - And don't take it personally. Just see the number, and realize if it is going up or down and make adjustments. Also, remember that water weight can fluctuate your weight up and down by several pounds in either direction in a given day. This is affected by eating salty foods, eating more carbs than you need, and dehydration.
Workout many times a week - instead of going to the gym a couple times a week for an hour or two. It's more effective if you want to get fitter faster.
Tie your workouts to your shower - I can't last longer than a day without showering before I start feeling gross and need to shower, always before bothers other people (I hope 🤞). The thought process becomes "I need to workout so I can shower". It's surprisingly effective at preventing workout skips. Please don't do this if you are fine being grungy, be kind.
Don't go hard all the time - This is a really easy way to burn out. Just have a couple of hard workouts a week and then the rest can be easy. Typically referred to as the 80/20 rule, where you go hard 20% of the time and easy 80% of the time.
Have a plan - Either a coach or a training plan of some sort, with a specific goal in mind. If you are training for a competition, a marathon, a triathlon, a cross fit competition, having a goal helps keep things on track.