How do you keep plastic leveling blocks from slipping?

OkSixpack wrote:
Could you take a rubber hammer and give them a tap to get them tightened up against the tire before pulling up on them?

Thanks for the suggestions everyone. I tried tapering the blocks, but they still slipped. I like the rubber mallet suggestion. I should add a rubber mallet to my supplies.

The lot I was parked in was newly paved and sloped downward.