Good question. It depends on a few things.
1.What's your goal, to make money, just get rid of the stuff, or a combination?
2. What's the going price?
3. What's the condition of the item?
Here's a few of the prices in our area...
paper back books $.25 or $.50
hardcover books $.50 to $1.00
children's books - the paperback toddler type $.10 to $.25, paperbacks or board books $.25, hardcover $.50
video tapes $1
DVD's $2 to $3
adult clothing $.25 to $50 for t-shirts, shorts, jeans - sweatshirts $.50 to $2.00 - jackets up to $8 depending on type, brand, etc (note: adult clothing usually does not sell well)
beanie babies $1
Children's toys that are clean and in good shape usually sell well and you can get a higher price for them.
Visit a few sales in your area for ideas.
When pricing keep in mind, what would I pay for this item? Also, mark it a few $$ more than the absolute lowest price you want. If you want $20, mark it at $25.
Sometimes to make things easier you may want to just price all similar items the same. For example my girlfriend priced a table of knick knacks at $.25 each. Some of them she could have gotten a $1 for, but her time just wasn't worth it.
Good luck.