Practice. I admit my danmaku is weak, I can pretty much do Easy Modo on IN without too much trouble. It's much easier (at least for a beginner) then PCB, I'm getting closer to the point where I can do Easy Modo in PCB, but I always end up running out of lives and having to continue by stage 6, I can't quite get Youmu right. Stage 4 used to be hard as fuck, but I practiced that more then any other to the point I can do it and probably only lose a life or two depending on luck.
EotSD seems easier, but then I found out after beating it on easy that easy modo doesn't count so I'll try that again on Medium and probably fall down on the stage 6 boss.
IN is my favorite so far, I played PCB first and liked it... but damn it's pretty harsh compared to the others. First three stages are a joke, then it just hits a wall. IN I've come to like more for the mechanics.
Finally, started messing about with Phantasmagoria of Flower View today, but I'm disappointed that it's not properly translated and despite having a pretty much complete script translated on the wiki, they always skip out the endings. So I don't know what happened when I beat everyone with Cirno.