1.Do not care about the Unique Unit much when you choose your leader - UU can only help you for a time but Unique Building and Leader Trail for the whole game.
2.If you are a Phi Leader, spam Great Scientists may not worse than Great Engineers - 3-4 Acadamys in your main cities will boost your

nearly 50% - Library + University

and you don`t want to spam more GEs than useful wonders, right?
3.Chariot and Axeman will help you pass "the Barbarian Age" and they`re much cheaper and more useful than the Great Wall but if there`s not copper or horse but stone, go for it.
4.If you have two religions, your can spread the non-state religion by spaming missionaries with Organized Religion to the civs you want to take ground from

when the war begins, you will findall your "friends" will help you in the "Holy War".
5.Catapults and Artillerys are not only valuable in siege but also defense.With the

advantage that provided by your roads and rails, collateral damage the enemy before or when they get to the city gate.
6.Take advantage when you find there`s a hill or a small forest

Promote 2-3 units with Guerilla II or Woodman II to protect your siege units against enemy hunting force.
7.Great General seems more like Great Medic to me

- I always warlord my high

to MedicIII, and that benefits my whole army.
8.Do not ignore your airforce, though the bomber and fighter are not so powerful as artillery but they can bombard several times before your arillerys get to fire position.
9.Do not place more then 4 units (except Siege Units) in one Tile, but also not less than 3 - large groups will suffer amazing damage from cats/artis while small one is easily ruined by counter-strike.
10.Use Nukes when there`s a must - do not use them in valuless nuclear revenge or something, use them when you saw enemy hordes in your force`s reach(either in a city or not, but not a city with shelter) - nuke them then finish them with your army - the most valuable target in a war is enemy units.