Just because you can hold onto an item doesn't mean you should. It can be hard to part with possessions, but if you haven't used that old ski machine in five years, it's probably safe to donate it to charity or to sell it on Craigslist. Go through everything to be sure that you're not hanging onto items you don't need or want.
Try sorting your items into a few different piles or bins. Put everything you want to donate in one pile -- old clothes, toys and sports equipment can be donated to your local thrift store. You can also donate clothes and toys that are in fair condition to a local shelter. In some cases, you can arrange to have an organization like the Salvation Army pick up larger items. Such service is usually free [source: Salvation Army]. In another pile, collect the items you've decided to throw out. Broken toys or tools can be trashed, and so can any miscellaneous clutter.
Your largest pile will most likely be the one that contains those things you want to keep. If you can find a way of storing these items that keeps them out of the way, then holding onto them shouldn't be a problem. However, if you find that your basement is still overflowing with stuff, you may want to go back and re-sort the pile to see if there's any way to further narrow down what you've decided to keep.
Throwing things away can be liberating, but inevitably, there will be things you need or want to keep. Check out the next section for tips on how you can store your belongings in a neat and tidy way.