Here are some important tips to keep your monofilament fresh so it lasts as long as possible and doesn't break when the big fish gets on*
(thanks to tackletactics.co.nz for these tips)
Sunlight weakens monofilament. Don't leave in direct sunlight when you are not using it. Pronglong heat weakens it. Never store your line (or your rods) in hot areas like car boots or a back car window, attics etc. The best place to store your fishing rods and excess line is in a dark, cool room, closet or garage.
Don't let your line come in contact with gasoline, oil, suntan lotion, or other harm-full chemicals. These cause monofilament line to break down or become brittle.
With proper care and normal use, monofilament lines ages. However, it's a good idea to periodically strip and replace old line. Two line changes a year should be sufficient - at the beginning and the middle of the season.
Last but most important be on constant guard against nicks abrasions or other weak spots in your line. You can detect these spots by running the line through your thumb and forefinger. If you find a rough spot, clip off the line above the bad spot and retie your bait. It's better to lose a few feet of line than to lose that trophy fish!
*this may have happened to the author more than once!