I asked this question a while back. Some people gave some good insight here and there.
http://www.conquerclub.com/forum/viewtopic.php?f=58&t=95547The biggest improvement I have seen in my own foggy games has been through the use of snapshots, and analyzing the log much more closely.
It's important to consider what your opponent can see of you. Keep in mind that is always different for different opponents. If you can leave 3s on your borders early in the game, your opponents won't know where you're dropping and attacking from, or where you're building up.
There are other map-specific keys to fog that are always good to keep in mind.
For example, on Waterloo, if you control all the artillery, you can see the whole map (except for one section) and no one else will be able to see the whole map.