Sweat stains are largely composed of salt residue that is left after the sweat moisture evaporates. On leather sofas, this typically leaves a white stain that is difficult to remove with many typical household cleaners. Vinegar is a natural solution for this problem. Diluted, vinegar is safe for leather and very effective at removing these kinds of salty stains.
Mix 3 parts white vinegar to 1 part water in a bowl for sweat stains on leather. In other words, mix 1½ cups of vinegar to ½ cup of water--this amount should be sufficient if you have just a few sweat-stained areas.
Wet a soft sponge or cloth in the solution. Rub this over the sweat stains. You should see almost immediate results. The white stains should start disappearing before your eyes.
Wet a new cloth or sponge in clean rinse water (from your kitchen or bathroom faucet) after you've finished cleaning the stains. Wipe the fresh cloth over the area to rinse off vinegar residue, then pat the sofa dry with a clean towel.
This solution can be used on sweat-stained leather jackets and other items. You can clean and condition the sofa as usual, if you choose, after using the vinegar to remove these stains. Prevent future stains of this kind on your leather sofa by using fabric throws. The sofa may carry the vinegar odour for a while, but this will go away. If you want to, you can speed up that process by opening some windows or turning on a fan to circulate the air.