1. Time Constraints
When your running around and doing so many things its hard to think of food and when you do, you don't want to spend an hour making a healthy soup or vegetable dish so it seems easiest to go out and buy takeaway. Something that is just as easy but MUCH better for you are frozen meals. These meals generally have under 500 calories and follow specific nutritional intakes. These meals take less than 10minutes to heat up in the microwave which is equivilent to the amount of time it would take for you to go out and get a burger. Canned soups may seem like a good idea also but these soups are highly lacking in nutrition and will leave you feeling hungry again soon after so try to avoid these despite their low calories.
When making food decisions, be concious of what you chose. Think about what food choices will benefit you in the long run rather than satisfy your cravings now. Chose a wrap over a burger, a salad sandwich over a meat sandwich, an apple over a biscuit, a fruit salad over french fries. By just slightly modifying what you eat on a daily basis, you will be able to see the results.
There are so many good foods out there to chose from so why just stick to the normal boring salad! Look up recipes and modify them to your taste buds so that eating healthy becomes something fun and that you want to do. Try recipes which require a lot of fruit or vegetables and look over the ones which have a lot of carbs of high saturated fatty meats.
4. Treat Yourself
All that being said, feel free to treat yourself every once in a while. Eating "bad" foods in moderation is not going to make you fat. I try and eat healthy whenever i can but sometimes you just have to treat yourself and get a burger. I think i would go insane if i didnt have a burger every once in a while or take something from my secret chocolate stash in my wardrobe. Remember, food is just another thing to enjoy in life and you don't want to spend your time worrying over everything you put into your mouth and dreading meal times. Make sure you enjoy the food you eat but keep into consideration the health aspect of things.
This is something i LOVE doing. I have been vegan, vegetarian, gluten free, no carbs, liquids only...everything you can think off. I love doing this because i feel like it brings such a variety to the way you eat and makes meal time much more interesting when you think more about what you eat. I don't reccomend staying these things for too long (YOU NEED CARBS!!) but to try it out for a week or so can really benefit your attitude towards food.
Those are just my tips for healthy eating, in no way am i professional nutritionist, this just comes from personal experience and what i have learnt along the way.