When the "CardistMag" was still running I did this interview with the "DYNAMITE" guys. I extracted for you just the part where they answered my question regarding how they create their flourishes. Hope this helps
Oliver: "I come up with new flourishes when I'm in my little cave hehe.If you just practice a lot you will come up with cool moves accidently.Of course you can also sit down and say "I want to create a cool new flourish", but I find that kinda hard."
Nikolaj: "What I usually do when I’m creating new stuff, is to find fun little original weird moves, and connect them. Try to sit down with a deck of card, and fiddle with weird grips, and consider “ What can i do from this position” that’s how I came up with a lot of my stuff. "
Dennis: "Sometimes I sit down and try to create a flourish,but most of the times it happens by random while I play around with the cards.It's hard sitting down and trying to create a cool move.Chill.Just do it and see what happens.That's my advice.
Tobias: It does happen by accident most of the time.If you want to create your own flourishes take the moves that you can do and combine them.Do a move to a point and then instead of doing "this" with a specific packet do something else.For example,if you usually go right with it,go left this time.Also,listen to some good music meanwhile.Don't think about it,just do it.