The best tip would be to not do sit ups. Instead learn the plank exercise, which is has so many more benefits.
Firstly , it strengthenes every muscle in your core (not just the front)
It burns more fat as it uses multiple muscles (crunches and sit ups do not)
Because it makes your core stability stronger you will gain more strength in other exercises such as press ups, squats and overhead lifting (crunches and sit ups cdon't do this)
You'll have better posture (crunches and sit ups force you into even more of a forward-rounded posture, which people suffer from anyway)
Planks are good for your low back health (crunches and sit ups are shown in studies by top spinal experts to be damaging to your back over time)
Spinal flexion is cited as the leading cause of disc degeneration, which is why planks and variations will do you a lot more good, help you burn more fat, gain more muscle and keep your back safe from injury.
Hope this helps.