Something that you can try that is not too expensive (of course other than chasing for vacuum leaks) is to change your fuel filter.

After removing the fuel filter, I usually take a white (or light colored) ceramic bowl and tap the filter towards the center of the bowl (ie... like you are trying to empty it out in the opposite direction of the normal flow). Be prepared for some residual gas to go into the bowl. Upon doing this you can get an idea of how much crap was in the system. You'll be amazed sometimes. Anyway, if it looks like there's a lot of crap in the system ... run some "high quality" fuel injection cleaner through it. Red Line makes a pretty decent fuel injector cleaner. This has helped clear up the "Fuel Trim" error in at least 3 cars that I've worked on. If it does help, then I usually suggest you take it in to have the fuel injectors professionally cleaned (on car cleaning is usually around $130 US). Best way is to remove them yourself and find a place that you can trust and have them reverse flush/clean them properly.

Hope this helps.