Expand, expand, expand, until you are stopped in all directions!
I've found that it is easier, rather than having cities produce settlers and units, to have one city producing settlers and one city producing units to guard the settlers. You should also know how to make a settler factory. Read Babylon's Diety Settlers in the War Academy to learn how to build a settler factory.
What I do to expand quickly is, first in your capital build a warrior to defend it. Build a couple more warriors to explore. Reaseach pottery (if you don't already have it) and build a granary next. Granaries are the key to settler factories, do not disregard these Now once you have the granary, build a settler. The city you build with that settler is going to be the unit producing city. Keep producing settlers in the first city and units in the second city (use these to defend the settlers) and in highly corrupted cities build only improvments and workers. Make sure you have alot of workers. Moderately corrupted cities can go through a rotation of unit, unit, settler, improvement, unit, unit, settler, improvement and so on.
Even if you can't build as many cities as the AI, the option of warfare is always open.
Btw, always keep about five extra settlers in your cities incase land opens up near your territory because of a war between AIs.
Don't bother with wonders, I mean in the higher levels you are almost never able to beat the AI to a wonder. If you're playing cheiftain- regent, go ahead pick up a couple wonders, work them into your rotation, but in the higher levels (monarch and above) its just to much time to be wasting to build a wonder and then the AI finishes it before you. Never base your strategy on wonders. This will cause you to fall into wonder fixation. Just focus on expansion.
Hope this helps.