There could be a couple of things causing this. First, clean your bars, your clamp, and your fork to make sure there isn't dirt or grease causing it to slip. Second, make sure that you're using the right diameter clamp for your bars. There is a subtle difference between standard and oversized bars and clamps, so you might have an oversized clamp tighted all the way down on standard bars, but it's really not tight enough to hold it. Third, you can try using a small piece of a popcan as a shim. Cut it into a strip that can wrap around the bars and inside the clamp to provide a little extra thickness.