What's a stupid kitchen trick? It's one of those things that someone shows you that make you slap yourself on the head and think, "Why didn't I think of that?" (Put down the knife first, though.)
I had that reaction the first time someone pointed this one out to me. You know how annoying it is when you're slicing things that are naturally a little wet -- potatoes, pickles, carrots -- and they stick to the blade of your knife? Don't knock them off: Use them to make uniform cuts. When you lift your blade and a slice is sticking, use that as a guide to show you where to place the knife for the next cut.
Simple, and kind of stupid. But handy.