It's all been said before re coasters, saucers, beermats dropping into the lap, we all know the problem so I won't repeat it. A slightly dimpled surface (to prevent vacuum forming) coupled with a non-stick PTFE coating (to prevent that dried-on-goo stickiness) should solve the problem.
Yes I know
the trick about salt. Useful after the event or if you're drinking tequila.
Also, a quick search on "coasters" throws up a corrugated coaster by one Egbert Chucklebutty. Coincidence or what?