My step-sis is bad for this. When she was pg with her bub, I gave her a heap of things. Not just clothes, but bigger items like play centres, rockers, bouncers, pram, high chair, wraps, blankets etc because I knew she had very little money. She never once said thank you. Now we're having another bub, she said she'd give us everything since we gave her so much, which is sweet... except I've seen her selling off her stuff on facebook websites. She has given me a couple of bags of clothes, but they are all the older/more worn clothes. I still thanked her, and I appreciate it as it's still a help, but I admit to being royally peed off that she is selling stuff that we got for her, and giving me the ones she won't get much for. Very rude.
On another note, with my first baby I did the photo-of-an-outfit thing. With the second, I tried, but it gets harder the more stuff you're given and with dressing 2 kids instead of one... this baby I think I've got no hope at all! I will try though. And I always thank people. Even MIL, who still buys my daughters things I would never dress her in. We have spoken to her about it politely (she buys very innappropriate/revealing stuff) but she still buys them. Probably on purpose, she does not like me much. We always say thank you, but those things go straight to charity. I'm not dressing my 5 year old in knee high boots and a leather mini skirt.