The way I look at appreciation is that it is a gift from God - to give to us so we can grow and prosper and be happy. It comes in many forms. It can come in your actions, thoughts, and feelings. Regardless of which avenue you take to express your appreciation, it will be well received and valued.
I agree with you. It is a gift that we all need to learn to use greatly. Either through our words, actions, thoughts, and feelings as you said.
Doing things for people not because of the gains is a great way to show appreciation and it will boost our self-esteem. That is because once that person is happy and satisfied, the joy will be transferred to us as well.
It is also true that we need to show appreciation with lots of sincerity, that way we could be appreciated as well.