I like to coat only half the rim of my glass with salt or Tajín so I can control how much I get with each sip. Pour salt or Tajín onto a small plate. Run a lime wedge around half of the rim, then dip the glass into the seasoning at a 45 degree angle, rolling it side to side until coated.
You can even do half salt and half Tajín rim using the instructions above.
I prefer little to no sweetness in my skinny margarita, but you can add agave nectar as needed to make it sweeter.