The best and all-round cheapest product The String Guy recommends to clean your steel guitar strings is readily available Naptha solvent, also commonly known as lighter fluid. Fast drying, as it evaporates while it cuts through dirt and grime, it is also great for bringing other steel parts on your guitar such as machine heads back to their original luster quickly and easily with a minimum of fuse or waste. Simply squirt a cotton cloth with a small amount of Naptha and rub it gently across the surface area. It's a good idea to clean the factory residue off new strings too.

Regularly cleaning your strings will increase their life, improve tone and add to your playing enjoyment!

You can buy it in quantity at your local hardware store in the paint department, or in lesser quantity in general stores and smoke shops as plain old lighter fluid like; Zippo and Ronsonol.

However, a word of caution: Naptha is highly flammable and is a known carcinogenic. Be careful when disposing old cloth rags that are soaked in Naptha. After all, it's lighter fluid! Combustion can occur if not well ventilated. Naptha is a very common solvent that is used in a wide variety of common household cleaning products in whole or part. Using it straight avoids residues from other compounds that it may be mixed with, but it does demand that you use common sense in its application and disposal.