There is a Science behind garlic that I cannot comprehend, but it works. Not the garlic tablets, the actual cloves themselves that you can pick up at the grocery store. When you want effectiveness, go with the fresh stuff.
You can try munching on them as they are (good luck), but I've resorted to cutting up a whole clove and putting it in a salad or mixed in with my food. At least one clove a day (if you need more, then by all means).
It's only been a few days since I've been trying this approach, but on the days that I've eaten garlic it seems like I don't have pinworms at all. If I don't eat it, I notice them.
That and I've been drinking hot tea every night before bed. I'm still battling, but the above seems to help tremendously while I'm waiting for my DE to get here.