I use mild dish washing or hand liquid soap. A few drops in some warm water and pull the line though a wet rag a few times. Try to avoid detergent based soaps, which acording to Scientific Anglers may remove the lubricants from the line's surface. I use Ivory, which is specifically recommended by Rio. The Scientific Anglers Line Cleaning pad is also excellent. I carry one of these with me. Most flyline dressings work best when applied to a cleaned line.