An easier alternative to soaps is to apply a generous amount of KY jelly to the sheath and penis and let it sit for 20 minutes. Wipe the area well with paper towels - there is no need to rinse. It has become my treatment of choice for cleaning sheaths - especially for horses that seem to swell up frequently. Many of these horses appear to be sensitive to soaps and detergents, even if they are rinsed well. A final note - take all necessary safety precautions when cleaning a sheath.
Many horses strongly object to the procedure.