First, season your salmon with whatever you like. I used salt, pepper, and italian herbs. Another good combo is salt, smoked paprika and garlic powder.
Now, here's the key to getting that skin to crisp up. Cut slits into the skin, being careful not to cut through the whole filet. This may take some practice.
Get a cast iron skillet super super hot. Add the tiniest bit of oil (I recommend grape seed oil or any oil with a very high smoke point) and using a folded up paper towel, spread the oil thinly over the bottom of the pan. Once that oil is almost smoking, place your salmon skin-side down. Now, wait. Don't touch it. Wait like, 5 minutes. You will see the salmon becoming opaque from the bottom up and wait for it to hit about half-way through the filet.
Carefully flip with a spatula. It should look like this. If it appears to be sticking, then wait longer. Nothing should stick to your cast iron if it is properly seasoned (oiled). Cook skin-side up for another 2 or 3 minutes on high (or longer if you like your salmon less pink in the middle).
Serve skin-side up if you like it crispy or skin-side down if you don't.