I don't know if anyone has mentioned this before (I'm new here), but I have found that taking practice LSAT exams is a superb way to prepare for the GMAT verbal. The reading comprehension and critical reasoning sections on the LSAT strike me as being harder than their counterparts on the GMAT.
I took one practice LSAT every day for a week before taking the GMAC sample tests, and as a result, the GMAC reading comprehension and critical reasoning questions were a breeze for me.
Right now, sentence correction is my only problem on the GMAT verbal. I'm using Barron's and Manhattan GMAT to tackle this hole in my knowledge base.
As for the LSATs, there are dozens of actual LSAT's for sale in major bookstores. They make for an excellent, rigorous verbal workout.