Yeah, outbuilding your opponent is tricky, because if you expand too fast you'll leave big empire that's weak and poor, and easy pickings for everyone else. If you go too slowly and defensively you'll be left behind. These are a few expansion tips that I find to be quite helpful...
-Chop rush every settler/worker: If at all possible you should make sure to chop one forest for every worker and two for every settler. You can't afford to loose those long turns of lost growth. If you don't have enough forests around a given city to do this you should build your workers/settlers elsewhere. If you don't have enough forests elsewhere you've probably positioned your cites poorly.
-Go heavy on the workers: You should have about 2-4 workers immidiately availible to improve the tiles around each new city as you build them. Don't found new cities until you've improved about 5-7 tiles around cities you already own, unless you need some vital resource right away or something. If you dont' have this many workers to spare you should be building workers, not settlers.
-Use your military: This can mean a lot of things. The most obvious application is capturing/burning enemy cities, but this isn't always possible. You can also pillage the countryside for gold to keep your economy going while you run a deficit and/or wait for those courthouses. Use sneak attacks to capture workers. You can also position a few archers on key forests/hills in your enemy's empire to bottle him up and scare him into wasting time building archer stacks while you expand.
Basicly you intend to beat a skilled opponent or high level AI you have to use every option available. This could mean defensive expansion, all out chop-rush warfare or whatever happens to be most viable. If you're "too cool" to take advantage of the best strategy for a given situation because it's "not your play style" or something you won't get very far.