The Ready State Virtual Mobility Coach is like having a virtual Kelly Starrett in your pocket.
Planks, planks and more planks! I am a big fan of the holds as a base core builder. Regular, side planks, shoulder bridges,etc. My physio always recommends working for 10 reps of a 10 sec hold; a bit of trembling is ok, but as soon as you lose your form or start to actually wobble, stop. You want to be solid for the full hold.