If you HAVE to move around, my personal priority for casting instants is:
0. SW: Death if you're below 5 shadow orbs and enemy is under 20% hp.
1. Mind Blast if you're below 5 shadow orbs (assuming 100 talent is CoP). Use on cooldown obviously
2. Devouring Plague if you're going to be moving for more than 5 seconds/if it won't mess up your normal rotation too much/if you're at 5 shadow orbs
3. PW: Shield self if you're going to be taking damage in the next 15 seconds and you use glyph of reflection
4. SW: Pain Spam for a little damage each GCD (it's better than doing nothing)
If you need to move FAST, start off with Angelic Feather to increase your MS. The glyph that increases your MS from mind flay is also extremely useful, but with COP you will only have the stacks for half your rotation. If you use surge of darkness talent you should obviously add priority to those procs, probably after mind blast.