Develop Gratitude
One of the simplest points of etiquette, saying "thank you" when someone has done something for you, has largely been lost.
Do not worry much about form: while a handwritten "thank you" note is a nice touch, it is not required in most situations. A thank-you can be delivered verbally, by phone, or even by e-mail - however, these have decreasing levels of impact, so consider which is the most appropriate.
Timeliness is also a factor - but if you're late, apologize for the delay and then express your gratitude (do not assume that it has been "too long" and that it's better to say nothing at all).
Also, be a gracious receiver: do not feel the need to trade favors. If someone does something for you, and you do something for them, it puts them in an awkward position (where does the chain stop)? If the opportunity to repay a kindness arises, you can act on it - but do not go out of our way to repay a favor to restore your sense of self-sufficiency.
When receiving a kindness, respond warmly an enthusiastically. Do not be lukewarm about a gift or a favor. Also, be careful not to say anything that could be interpreted as derogatory or critical of the gift.
Some people are in the habit of attempting to aggrandize the importance of a gift by saying something like "you shouldn't have" - which actually suggests to the other person that they have done something wrong, or that the gift is unappreciated.