Mild soap (not detergent) like Ivory liquid and water. Rinse well, dry with a clean dry cloth. Dress with any of the available commercial line dressings (I've used Loon, Wulff and SA, all worked fine) If you have an Orvis Wonderline or an SA w/AST, the mfr used to recommend against dressing, just clean them. Both companies now sell a product specially designed for these lines, but I haven't tried either one.

Taking care of your lines will keep them in good shape for years. I have some lines that are 10 years old and still OK. The ones I use a lot (50-60 outings/year) still last for at least three seasons. When I fish highly alkaline water (spring creeks, some lakes) I always rinse the line with cool tap water as soon as possible (assuming the tap water is not itself alkali, of course) Take care not to leave them in the car on a hot, sunny day. That film you notice on the inside of your your car windows is from the plastic on your dash. The same thing happens to a fly line exposed to intense heat, the plastic coating breaks down.