I'm terrible at that stuff.

One thing I do try consciously to do is to thank people for things, or to tell them when I like something they did/made or enjoyed some time we spent together. That way I'm not offering an evaluation of them as a person, which I find difficult (because what if they don't feel the same way, because who cares what I think anyway, because what if they think I'm weird and stupid?), but I still show some appreciation.