If you're using a stickerless cube, try a stickered one. It might provide a slightly better grip. (Or it might not, but you won't know until you try.)My hands are always slipping off the cube
Also, I've heard Mats Valk uses baby powder (?) to improve his grip. Might want to experiment with that too.
Practise it a few thousand times to build dexterity in your fingers.I've never been able to do U2 with index-middle, as I just can't stretch my middle finger
Alternatively, thumb-index/index-thumb double flicks (with different hands) are much easier to get used to, but harder to execute consistently. The only fast person I've seen using them is Mats, and he doesn't seem to use them often either.