Here’s a quick and simple (yet, no less important!) tip: Don’t cut off their feet!
Nothing ruins an otherwise fine portrait more than cropped out feet. Everything else about it can be stellar. Lighting, composition, outfit, perfect smile… doesn’t matter. It is all ruined if they look like they’re standing on two peg legs. And really, it takes no time to just scan around the edges of your viewfinder and make sure everything is in there.
Cropping people halfway or three-quarters is fine. But if you’re going for a full portrait, go full portrait. Don’t forget the feet.