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The O2 sensor after the cat is not used for fuel trim, its sole purpose in life is to make you fail a smog check when your cat is busted.
If your vehicle is consistently reading lean, look into clogged fuel filter FIRST. If you have never replaced yours, and your vehicle has more than 15k miles or so, replace it. Your fuel pump will thank you, as will your performance and mileage.
AFTER replacing your fuel filter, if you still have enrichment trim that is consistently more than +3, rent a fuel pressure gauge with purge valve from your local Vato Zone or Checker etc. (it's like $70 but you get 100% of your money back when you bring back the rental) and make sure fuel pressure at the rail is within spec for your vehicle. If it is within spec, also check flow by pressing the purge valve while idling. The engine should not die easily, if it does you may still have a flow problem with your fuel pump.
Does the vehicle tend to "run out of energy" on hard acceleration above a certain RPM? Like for example does it accelerate strongly when floored, up to 2500 rpm and then kinda not get any more powerful as rpm's increase?
FWIW - gasoline burns most completely at a 14.7:1 ratio at sea level, but releases the most btu's per pound at about 12:1 - so under the RIGHT circumstances, running slightly rich can actually increase your mpg.
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