I fought with gradual slippage on three posts and four different clamps-- and tried everything from grease to anti-seize to fixing paste. Over the span of ~15 miles, the post would slip as much as 1/4". My knees were not happy. Finally went to a Unicycle-style clamp (like the one mentioned right above the post) a $16
Gorilla Park clamp from Taiwan. Absolutely no slippage.