It's a good idea to take that practice test so that you can see what your actual strengths and weaknesses are. That's going to determine the kind of work you need to do in order to improve.
In terms of non-GMAT reading sources, if you want to improve your Reading Comprehension (RC) skills, these are good sources:
*
http://magazine.uchicago.edu/ - particularly articles in the "Investigations" tab
*
http://harvardmagazine.com/ *
http://sciam.com/ (This can get a bit too casual for the GMAT, but it's probably worth including if you don't like science passages on the GMAT.)
The other sources you listed can be good for picking up on grammar, though I often find errors on such sources as well. NYTimes publishes an interesting blog periodically about mistakes that were made in articles recently published on the site. Here's a link to the most recent entry:
http://topics.blogs.nytimes.com/2009/08 ... nt-help-3/So, take that practice test and see where you're actually struggling. You don't mention what resources you're using to study, but it's also important to make sure you have something (books, usually) that actually teaches you how to do each of the three different question types (SC, CR, RC). The official guide (OG) books are great for practice, but they do not actually teach you how to answer the questions. So, if you don't already have some books to teach you how, you'll also want to get some.