Good information so far, you also will want to wipe the inside of the cap as well as the rim where it screws onto, that will help opening next time. The funnel looks to be the best answer
Set the can on the floor between your feet so that it can't turn. Grasp the lid with the channel locks while pushing down on the lid with the palm of your other hand.
It's also a great stretching exercise for your hamstring muscles.
+1 on both counts for me, but I always cringe when my thumb is in front of that blade. I also clean the threads with a solvent before closing when i have time.