Be sure to hold the thing steady when banging the sticks--it's easy to wobble the slate once the stick goes down. They need to see the frame where the sticks close--if it's moving it could be blurred. It's hard to keep it perfectly still, but don't dip it 6"!! Also, don't rush the whole process to save film--be efficient, but get it right by speaking slow enough to be properly heard and seen. The cost of a second of film is less than totally screwing up a take and needing second sticks or (**gasp**) third sticks. You look more professional if you're under control and get it right even if it takes a hair longer. Slate fast, but not TOO fast.
Lastly, for you 1sts, operators, dps, whomever, please don't humiliate a second assistant who needs to work on his slating skills!! This post started with someone who didn't know what they were doing wrong--obviously they got yelled at, but didn't know why. What did yelling at them accomplish? If you want to solve a problem with your crew, present a solution instead of inflating your ego. I think it's best to have the 1st quietly tell their second how to do it better (or correctly). Humiliating them only makes you look like an ass to the rest of the crew (and them!!).