It definitely depends on the alg for me too. For F, I always do the same finger trick as been stated above (on URF flick it down). For F' sometimes I'll use my thumb, something I'll use my index finger. If I'm doing the T perm, I'll do F' with my thumb, if I'm doing Jb (the T one) or Y, I'll do F' with an index finger push from RFU to UFL (You can thank Rowan for this trick). As for F U, I'll often do F as normal then the U will be like doing the second U in sune. I put my index finger on RFU and push it over to FLU. Sometimes I'll do F then I'll do U with my ring finger on BRU and pull it over to FRU (depends on the alg as well). For F2 (although I don't think of any alg I use that has it, except in 2x2) I'll position my right index/middle and do a flick similar to U2.