I used to just clean out my closet and donate stuff to a charity and/or a thrift shop. Now, unless something is really worn out or I'm pretty sure it won't sell, I go ahead and photograph it to list on eBay.
As an example, I had this t-shirt for years because I liked the design a lot, but it was always a bit too long for me, so I only ever wore it a couple times. Finally, I went ahead and sold it for $11.50. Not a huge amount of money, but I don't sneer at $10 here and there! And honestly, I make my way on eBay with a series of small sales; it's rare that I have one item that is a big earner for me.
When I read advice to new eBay sellers, one of the most common pieces of advice I see is to start out by selling some stuff you have around at home and no longer need/want. I think it's great advice. You have nothing to lose as long as you make sure that your starting price/shipping price covers what it will cost you to mail the item.
A lot of people seem to shy away from selling clothing, but I think as long as you have clear photos and detailed descriptions and measurements in your listings, clothing can be a good thing to start out with selling. If your kids are outgrowing their clothes, what about selling some children's clothing? Kids' clothes can sell well, especially in lots as a lot of buyers want to stock up on a particular size. Toys are another good seller.
If you have DVD sets, they can sell well, too; people love to buy full seasons of their favorite TV shows. Household items can also do well: I know that Suzanne has sold a number of picture frames, and I have sold a number of small display easels. There are people who collect Corelle dishes and all sorts of things that you might never think twice about giving away. You just never know.