I'm not on Diety, but I do beat Emperor regularly, and my favorite method of victory is Domination. Basically it all boils down to commitment. If you want war, then build units and war. Don't mess around building culture and all the buildings needed. Build units and go into battle. You'll also want to tech to be able to have an advantage in battle. You may stick to 2-3 primary cities at first, as you'll be taking over cities as you go. Once you get a good force going, then go back and build up your cities and keep upgrading your army.
If you want to start taking cities at turn 50, then get to composite bowmen and follow up with getting to Crossbowmen. You'll find that the ranged units are really good in war, so you'll want 4+ ranged attackers when you go in after a city. I tend to ignore catapults and ballistas, as they get killed way too easily and hard to get into battle. Cannons and Artillery are very good, however.
You may also find on continents and maps with lots of water, that Frigates with some privateers make great units to take coastal cities. 4-5 Frigates and a couple privateers can rage havoc on your enemy if they have coastal cities.
Besides committing to your war, you also must focus fire your targets. If you target the same enemy 3 times and fail to kill it, chances are, he is going to use an upgrade to heal it 50 points. It is better to finish off 1 enemy, rather than weaken 2, because they may heal up on you.
Before going at a city directly, you'll want to get all your men in position, so in 1 move, you can move in and attack with all your archers/crossbowmen. Coming in at different turns slows the attack, which also means your guys take more damage.
When upgrading your men, which should preserved, and not treated as disposable units, make your ranged units upgrade either for rough or flat terrain. After 3 upgrades, you can make your men attack from 1 more range, avoiding city retaliation, or you can get double attacks, allowing for double the hits per turn and twice the experience. I typically will give my first 2-3 range, then the last couple the double attack. The same goes with Frigates, though I like to have my privateers to pick up heal out of their own territory, as every time they attack, they get hurt.
As you get later in tech, you will need to have a few fighters to defend your cities and naval fleets and don't forget about nuclear warfare. A single nuke can devastate your enemies forces. Nuclear subs and Missile cruisers can both carry nuclear missiles, so arm your fleet with a couple. If you see the enemy come at you with a 20 man army, 2 nukes can eliminate them in a single turn.
Many of my late ending games are done by taking a coastal city, puppet until it can be annexed. Then I buy a paratrooper/landship/tank, anything with a long range, then sending 2-3 nukes at the final cite(s), and sending in my paratrooper to take the city next turn. It seems cheap, but it is very effective.
Sorry for the rambling, those are just some of the thoughts that come to mind when you decide to go to war.
