I have noticed lately that when playing a song with a more aggressive attack on a bunch of chords (Who Do You Love, specifically), my pick slides from a nice position with the pointy end here it should be.

However, after a short while, it slips, and i'm either playing with the side of the pick or my thumb, both of which make it sound like crap.

What is going on, and how do i grip that pick?

Thanks in advance.