If you live in a neighborhood, ask your neighbors if they are interested in having a community garage sale. If you have more people involved, you'll generate more stuff (and interest from outsiders!), so you'll likely make a little more if it's a joint effort.
A site like ebay.com is good for smaller, lighter items, like clothing or small toys, that can be easily shipped. And don't forget about all those books you have lying around! Half.com (a company of ebay) makes selling your used books easy. All you have to do is enter the ISBN or USC number, and you're on your way to making some money!
Get the kids involved by having them choose a few of their own items they'd like to try to sell. Help them write the descriptions and take pictures to post online. They'll get a kick out of seeing their own stuff on the Internet, and will be equally as excited when someone wants to buy it!
Keep in mind that a second-hand store might not accept all of your items — the condition and quality of the items matter. If you have several stores in your vicinity, shop around to see who will take your stuff and give you the best price. Ask your kids to put a price on the items they'd like to sell. How much do they think it's worth? What's the lowest price they're willing to sell it for? It's a good lesson in learning the value of their possessions and how to compromise with others.
Donating is also a great lesson in generosity and selflessness for kids. Ask your child to set aside a certain number of items she'd like to donate to charity. Talk to her about how her contributions will help someone in need, and ask her how helping others makes her feel.