I've been playing for about 3 years, I play metal and my picking is driving me ****ing crazy lately. I use dunlop jazz III picks and tend to alternate/economically pick everything. I can play pretty fast now, but every time I play fast riffs, the pick slides down my thumb and index finger until it's almost in my palm. Any tips?
Ibanez RG370DX
Epiphone SG
Simon & Patrick Songsmith Acoustic
Crate GLX 65
EHX Metal Muff
Try some other picks? Try holding it at some different angles too.
Jackson DK2M
ESP LTD M-200FM w/Tone Zone + PAF Pro
Ibanez RG7321
Digitech Whammy IV
Digitech GSP1101
Furman M-10 LxE
Peavey 6505
ISP Decimator
Try looking for picks that have scales where you grip it... Might help for some grip.
don't use slimy picks!!!

picks that are cast from gel suck for metal and they get slippery when wet...

use powdery picks, like yellow turtles by dunlop

know what i mean?

also, use need a thicker pick, between medium and think, for metal, not thin...

dunno what you are already using, but i hope this helps!

check out the main riff for Bleed by Meshuggah!