Correcting others in front of others, doesn't work. It's not encouraging or polite. But if you really need to correct, ask them first, if they would like some feedback. If they agree, then, do it when no one else is around, do it in a kind...

"I should exercise more."  "I should eat healthier."  "I should do more."  The word "should" isn't negative but the way we use it, has a negative tone. Think about replacing "should" with something that's more encouraging and empowering.  How about, "I would like to...