This is one of the ways to do it. Some of the states pay really well if you give them recyclable materials. In certain states, you can actually get around 20 cents for a simple can of soda. So if you have a lot of soda cans or plastic bottles lying around, you can make some cash. There are other recyclable materials as well and you can check with the local recycling department for more details.
Some people call this process upcycling as well. The idea is simple, use the useless product as something else. For example, you can use old glass bottles as vase for flowers. Add some color to the exterior and you may have a fantastic new vase for almost free.
Similarly, you can use old and useless fabrics and other products in a similar way and use your imagination to bring them to use. You can certainly add a lot of value to your interior for very little this way.
Ebay is the answer to your prayers as well. You can sell just about anything on ebay and you may find a buyer for an item online that may appear to be useless otherwise. You can also make use of other sites such as amazon.com for a similar result.
It is quite possible that you may find a rare item in the clutter that you have. Go through it and if you find something that may be of value, you can always get the value checked. There are evaluators available who can let you know the value of your item. You will probably strike out most of the times but you never know and can hit a grand slam as well.