Have some build up to set up the relationship that is about to get destroyed. Have some good hidden ntr in it. Have some gradual corruption progression that shows not only on the sexy parts, but how the heroine treats MC.
Frankly its not that hard, but if you want to churn out games quickly, the first point gets forgotten. If you miss the first point, you have no foundation and the whole thing falls apart.
For me, and I know this one is pretty subjective, I like to have no choice in ntr. You give me choice whether to get ntred, and it stops being an ntr and becomes a ntrs. I want a story that is crafted with purpose and good storytelling, that requires the game to be very much on rails.
All systems where there is variable corruption, where you can get the heroine to max lewd by farming a lewd goblin encounter, or other forms of "freedom" just dont go along with good ntr. All storytelling and corruption cohesiveness goes out the window.
There is a reason why all the best ntr I can think of are all very much without player choice, or maybe just with choice of antagonist. Clipick games, ntrpg 1 and 2, henteria games, dark hero party - these are all games that I would put at the top of ntr and they are all very cohesive stories with the player being swept by the currents rather than having his own influence matter. That doesnt mean he doesnt try, its actually that futility of own actions that again, makes ntr hit better imo.
I see people often whining about having no choice, and while I agree with a lot of porn game genres that choice is nice, with ntr it ruins the whole thing. At least for me. There are some exceptions, but those are rare and very finely crafted (think mirage defection).
