I'm by no means an expert but I'd second Coffeeholic's suggestion of atleast 3 left-handed bats in the lineup. I usually try to have a lefty batting leadoff in Wrigley, one in the 3 or 4 spot and another in the 6/7 (+clutch which is conveniently ball park independent) area. I almost never manage to pull it off, but thats my goal regardless of my opponent's pitching. It's far cheaper for an opponent to pickup a 9R starter capable of shutting your lineup down than it is for you to pickup a significant left-handed bat midseason to counter the move. Somebody like Beckett is around 1.5 and there are guys close to .5 that will do well against an all righty lineup.
I know how hard it can be to find a lefty bat that won't suffer too much in Wrigley. Casey and Kotsay are two of the best I've had but if they're gone that's not much help. Helton and Hafner have also done well for me but are expensive (Helton's numbers certainly weren't 6 million dollars better than Casey's in my opinion). You might want to take a look at Gload if he's out there.
That said, Buck did well for me in Wrigley (41 homeruns though there were also 2 Coors in the division to help him and a US Cellular). Cabrera's been ok so far through a fourth of a season (good power, mediocre average but he's been improving lately). Clemens and Oswalt both won 20 games for me, era's of 3.71 and 4.15 respectively (once again 2 Coors contributing a bit I'm sure)