Or use a grinder/dremel tool to remove the worn out tip exposing fresh hex stock underneath. Just don't overheat hex key while doing this. This will extend their life by at least 3X.

One other hex wrench tip: not all hex wrenches are created equally. Cheaper tools are much softer and are much more likely to round out hex recesses while rounding the corners of the tool at the same time. Quality hex keys are a good investment to make.