here's how I do mine. In my
error log, I had columns for answer choices (A), (B), (C), (D) and (E). Each time I go through a question, I go through all the answer choices. For each answer choice I can convince myself 100% it's wrong, I put a cross in the box. For each answer choice I can't convince myself 100% it's wrong, I put a circle. When I arrive at my answer choice, I put a tick if I'm 100% sure, if not, I put the tick enclosed by a circle.
Use whatever notation works for you. The whole exercise is to improve your ability at reading the answer choices and being able to sift out what's the dodgy answer (sometimes it's just some words in the answer choice that changes the entire meaning) and what's the right answer.
Granted, it's a painful process, but do this for a week or two, and you should be able to see some improvement. Once you improve your hit rate, you can move on to improving your speed. Work on one thing at a time. The GMAT requires a very systematic and tactical approach.
Also, for each question, include a column where you write in the question type. Is it a assumption, inference etc. Look at the mistakes you make. Are you failing to answer correctly for a particular type of questions (you might be failing miserably due to assumption type of question etc). Usually, a person is doing poorly on 1 or 2 types. I haven't seen anyone with a general mix yet.
And as for more questions to work on, you can try the LSAT CR as Paul suggested. I did not work on LSAT as there're some questions in LSAT CR which is not standard on GMAT. But it's good to have a go. You can also post them on the forum for discussion if you're not sure how to answer them. For myself, all I used was the OG, the Kaplan verbal guide and the Kaplan 800. I bought the OG second hand, and borrowed the other two Kaplan guides from the local library. Make a search, you'll be surprised what they have in the library.
Finally, keep plugging away, you'll see the results sooner or later. Good luck !