Your abs are the same as every other muscle. After training they need time to recover, this is when they get stronger. So if you are able to do sit-ups or an ab routine every day you probably won't see much progress as you aren't working them hard enough or giving them recovery time
Your abs are built to work in a number of ways and to contract in different directions. To train them properly you need to work them in different ways (instead of just doing sit-ups and crunches).
For example crunches, reverse crunch, plank and ab bikes works the abs in 4 different directions
Breathing properly during exercise is important as it allows you to contract the right muscles at the right time and makes the exercise more effective. A common mistake is to hold your breath whilst doing ab exercises.
Instead you should concentrate on breathing in during the relaxing part of the exercise and breathing out on the contraction part (when you tense the muscles)
Abs should never be the first thing you train in your workout. This is because they are extremely important in stabilising your core during all exercises (particularly during strength training).
If you exhaust them at the beginning of your workout then you are risking them being tired and not as strong for the rest of your exercise and this can increase the chance of injury. Always do your ab workout at the end
To improve and see results you need to keep progressing your exercise. If you are doing the same ab routine, with the same reps or the same exercises you won't see any changes. To progress you can increase the reps, increase weight or make the exercise more difficult.