a mortal and pestle is the best way, but if you dont have one, just hit it with a side of a chef's knife a couple of times, this is just to release esential oils and flavors.
I've heard of bruising leafy herbs like sage or basil, but never something like thyme. Try stripping the leaves and mincing them like you would garlic, with the flat side of a chef's knife.
I do it in the palm of my hand. Put the herbs in there and just roll them between my fingers. I kind of use the palm of my hand and my fingers like a mortar and pestle.
When it comes to fresh thyme, I just put in branches when I start cooking, then pull out the stems before serving. I agree with everyone that crushing fresh thyme seems not right. The leaves are so small. But just rub the branches between your palms and toss them in.