What Not to Say About Someone's Appearance

Don’t say: “You look tired.” 

Why: It implies she doesn’t look good. 

Instead say: “Is everything OK?” We often blurt the “tired” comment when we get the sense that the other person feels out of sorts. So just ask.

Don’t say: “Wow, you’ve lost a ton of weight!” 

Why: To a newly trim person, it might give the impression that she used to look unattractive. 

Instead say: “You look fantastic.” And leave it at that. If you’re curious about how she got so svelte, add, “What’s your secret?”

Don’t say: “You look good for your age.” 

Why: Anything with a caveat like this is rude. It's saying, "You look great―compared with other old people. It's amazing you have all your own teeth."

Instead say: “You look great.”

Don’t say: “I could never wear that.” 

Why: It can be misunderstood as a criticism. (“I could never wear that because it’s so ugly.”) 

Instead say: “You look so good in skinny jeans.” If you slip, say something like “I could never wear that…because I wasn’t blessed with your long legs.” 

Source: 18 Common Phrases to Avoid in Conversation