1. Regularly clean your line. Sometimes the line will become soiled with a variety of contaminants and will need some attention. During this time, as you run the line through a moist cloth, it is a good time to inspect the line for cracks and defects in the fly line. If you need to use a mild soap, make sure to run the line through a second time with a wet cloth to remove any additional soap.
2. During the off season, or when your line will be stored for long periods of time, pull the fly line off the reel and loop it in large loops and hang on a hanger. If fly line remains on your reel for long periods, it will create memory and can crack if it is tight on the reel. It will take some time before the straightens and is usable, especially in cold weather. Just before I head out to the water, I will pull off the first 30 to 40 feet of fly line and pull it through my hands, creating some warmth from friction and will straighten the line.
3. Next, whenever you cut into the fly line to tie on your tapered leader, it is a good idea to place a tiny drop of waterproof adhesive on the cut end of the line. A fly line can have a hollow core and if left open, can cause floating line to sink. Water can seep in and make presenting the fly unnatural. A tiny amount of waterproof adhesive applied with a toothpick will do the trick.
4. Lastly, be aware of how much you cut the fly line. When new leaders are attached to the fly line, especially weight forward lines, the amount cut can effect how the line casts. It has been my practice to attach a 4 to 5 inch section of 20# monofilament with a nail knot to the fly line and tie a perfection loop on the other end. This way, I can easily change leaders with having to cut into my fly line. Also, it's a lot faster.
In this era of budget consciousness, I can't afford to replace fly line annually. It may take a little more effort, but it is well worth the time to maintain the most important part of your fly fishing arsenal, the fly line!