Burn more cities... Keep the few that are worth keeping... holy cities, wonders, key resources, well developed and good land.... First, you can roll through on offense faster without having to protect every city you take, and you don't totally trash your economy by keeping a bunch of "less than optimal" cities. AI's will build cities ANYWHERE. Many of them are pretty worthles... why keep them.
Also... you might want to consider caste system over slavery if you are going to build the mids and run rep. You can get specialists going without having to build the improvements. It can really jump start the beakers.