If you have furniture that is sweat stained, the stain should lift with white vinegar. Dampen a cloth with vinegar and work from the outer edges of the stain toward the center. This will keep the stain from spreading. After you have finished with the vinegar mix a teaspoon of dishwashing liquid with warm water until it is really foamy. Take the suds only and rub the furniture with them using a rag. Work from the outside of the stain inwards. Change the water as needed.
Source: http://www.cleaningtips.com/tips/house/upholstery-cleaning/cleaning-sweat-stained-upholstery.html