i don't. I prefer to thank them in person. The only time i send one is when I know i won't see them again like leaving a work place.
i dont cos i always just thank the person immed. but sometimes i do send a sms to thank again after that too. =) but thats the most i do.
Say thank you, and ALWAYS, ALWAYS send thank you card, especially if I didn't open the present when the gifter is present.
It depends on the person I guess and also where they are. It somewhat feels arkward for me to write a Thank You card or note.