My kids pretty much pay for their own clothes unless I see something in ds's size at the thrift store that I can't resist. They also pay for their own cosmetics, a fair amount of their own food, music, video games, and movie rentals, and their transportation.
When dd was working and I wasn't, I didn't charge her for rent or ask her to pay a share of the grocery bill, but she voluntarily gave me a lot of presents that I couldn't normally afford and/or wouldn't have bought for myself: clothes that express my personal style and aesthetic sense rather than the cheapest crap that almost fits, a nostril piercing I'd wanted for over ten years, the first restaurant meal I've eaten in over six years, and almost weekly "coffee shop hopping" and trips to local art galleries. She was still able to save up enough to cover tuition for two classes last summer.
Now that I'm working and she's a full-time student, money is even tighter. ds generally buys her little treats with his pay when he works: food treats that are a bit more exciting than potatoes and dried beans, a movie rental, and last night they shared a tub of Manic Panic. They were planning on getting a roll of film developed as a surprise for me, but he didn't have quite enough money.
I'm right there with you about not wanting to charge my children rent, even though dd will be eighteen in March.