Wood rots because of natural factors, primarily water. Placed outdoors, nearly all wood will eventually rot, but there are ways to help prevent it or slow the process drastically. Steps can be taken before fence posts are installed, but already-existing fence posts can be treated to slow decay. The largest problem with treating pre-existing fence posts is that the part of the post that sets below ground level will still rot at the normal speed. If possible, remove the fence posts from the ground and reinstall them after the steps have been taken.
Dig a hole with a shovel 10 inches deeper than the fence posts need to set. Fill the bottom of the hole with 10 inches of a medium-grade gravel. The gravel will allow water to drain from around the posts more efficiently. Tamp the gravel down with the bottom of the fence post so that it settles in place and will be less likely to shift once it's set in place.
Fill a bucket with a copper-based wood preservative. Place the bottom of the fence post in the bucket and allow it to soak for two or three days. Wear rubber gloves and use a paintbrush to coat the post a few inches higher than ground level. Brush the solution onto the post three or four times a day so it can soak in.
Cover the bottom of the post with an asphalt-based foundation sealer after the copper-based preservative has dried. Let the foundation sealer dry for a week.
Spray the entire post with a waterproof wood sealant. Spray the post evenly with a garden sprayer, allow it to dry for an hour, then add another coat of sealant with a paintbrush.
After the posts have been set, check twice a year to see if they need to have another coat of waterproof wood sealant applied. Splash the post with water. If the water beads up, the post is still protected. If the water begins to soak into the wood, reapply the sealant with a garden sprayer and brush.