LPT: The most heartwarming compliment you can give to someone is to compliment them on something they've been working hard on

I had a friend that kept a private note in his folder saying, "stand up straight, speak louder, be more confident." I was in the fortunate opportunity to know him in high school, university, and later on, work - but always as his senior. I just happened to notice the note one day and honestly, I really did think that he seemed like he improved in his confidence levels. So I told him exactly that. He seemed really happy.

Like any compliment, you should only give it if you really mean it, but compliments regarding something that people invest a lot of effort into make all the difference. It's just a little encouragement that can make them feel their hard work is not for naught. In terms of positive conditioning, it makes them more likely to keep wanting to pursue their goals.

There was one study (can't recall which one) whereby one set of children were praised for effort and work ethic, and another were praised for an absolute level of intelligence. The ones who were praised for effort could face setbacks significantly better, whereas the ones who were praised for absolute intelligence were distraught when they were met with actual failures. So in knowing this aspect, praising someone on a quality of their character that they can change (for example, honesty or optimism) can be more meaningful than something they have no control over (for example, natural beauty).

Tl;dr - a compliment is meaningful to a person if they worked for it, and is also more likely to be more meaningful if it is within their own locus of control.