You are NOT
Building enough cities
Making good use of workers
The AI is at a disadvantage on Warlord and is seriously crippled on Chieftan. On regent its all even.
You must produce enough cities to have the money for research. You must use workers to improve the land so you will get the shield for building city improvements and military units.
There are a lot of helpfull threads for this on the Strategy forum and even some on the General forum.
Here a few of the those presently active that should help a bit.
REX - An Essay
http://apolyton.net/forums/showthrea...threadid=59146
The Virtues of being Industrious....
http://apolyton.net/forums/showthrea...threadid=50963
How to automate workers correctly? Late game. SO they don't mess things up.
http://apolyton.net/forums/showthrea...threadid=59554