One of the best ways to increase the life of your water filter is to start with the cleanest water that you can find. While a babbling brook may seem cleaner than a pool of standing water, this is not necessarily so. Flowing water stirs up dirt and sand on the bottom of the water, making it easy to pull that into the water filter. For the cleanest water, choose a pool of standing water, and don't let the hose touch the ground. If this isn't possible, dip some water into a pan and let it set for at least an hour before running it through your filter. This will give the sediment time to settle to the bottom.
Some of the most common problems with water filters are also easily preventable. It's important to be gentle with the filter. If you drop it, it may appear fine and continue to work, but the inside may develop small cracks. The microorganisms that you're trying to filter out can easily pass through these cracks without you knowing it -- until you get sick.
If you're camping in cold weather, it's important to remember what happens to water when it freezes. If your water filter freezes while it's damp, the water inside can expand, causing cracks and leaks to develop. Tuck the water filter inside your clothes during the day, and sleep with it at the bottom of your sleeping bag at night.
Finally, make it a habit to keep your water filter dry. When you're not using the water filter on your trip, carry it outside your backpack in a mesh bag. When you return from your trip, flush the filter with a weak bleach solution and let it dry thoroughly before packing it away. This will prevent bacteria from growing inside of the water filter.