Just finished writing an overly long, detailed explanation of how I used to do it with the M16 I carried for 18 mos. Don't know exactly what I did, but lost it all.
Short version. Lube it to hell with TufGlide. I don't move finger to the flipper. I just press my finger hard against it, on the top, until ready for my "move." From horizontal, blade up, can open/lock easily. From vertical, takes my strongest move to get it all the way up to lock. From any other position, can quite easily open to lock.
Before I lubed, used to use the wrist flick with the flipper. Haven't needed to do that in months. 3-4 treatments, some working, and it should go well. Of course, am talking about the largest of the M16s, the 14. Those with a lighter blade may need the wrist flick in combo with the flip.
IMHO, can get the M16 into action quicker than anything except the Emerson wave.