**Fly lines are semidisposable, which is all the more reason to take care of them. The majority of lines are made of an outer layer of PVC or a polymer over one of several core materials. Chemicals, excessive exposure to heat and sunlight, and abrasion can damage them.
Solvents damage fly lines pretty quickly. Bug dope is particularly harmful, so if you spray or rub it on, clean your hands well before fishing. If you're fishing from the back of a boat, keep an eye out for problems, including oil and grease on the outboard and steering cables, as well as leaky gas tanks.
Keep fly lines out of the sun as much as possible when not in use. The plasticizers that keep the line coating supple break down, causing the line to stiffen and crack prematurely.
Standing on your fly line can cause problems beyond ruining a cast. A rough nonskid boat deck can abrade a fly line instantly. Also, sliding your feet over the line can twist it.
Keep your lines clean. If you notice the line "sticking" in the guides, stop and clean it. Lacking anything better, wrap your shirttail around it and pull the line through it a couple of times. For lines with self-lubricating coatings, microabrasive pads (such as those made by Scientific Anglers) are great for removing caked-on algae, salt and other debris. After the day's fishing, do a more thorough job. Wash the line with soap and warm water, pulling it through a soapy rag a few times. Rinse it well and put it back on the reel.
Line coatings and conditioners are needed less than they once were. Today's fly lines are largely self-lubricating, and removing any built-up surface crud restores them to like-new condition. If you are particularly fond of using a commercial coating/cleaner, I recommend sticking with one made specifically for fly lines. A chemical made for another purpose may be ineffective and, worse, damaging.