I guess this is a question that puzzles many of us, and I have not come across any quick solution to improve RC.

well, here is a strategy that I believe will work if you have enough time. The most important aspect for improvement in area is genuine feedback. If you know your current level, you can try a little more difficult level and achieve it. However, If you don't know your level, you keep fumbling around to figure out why some times it feels like piece of cake, and other times it feels like biting a bullet.

The below website provides various features that could be used to improve RC. Here is my explanation on how you can utilise it to your benefit.
http://www.lexile.com/

1) They have a tool to measure "Lexile measure" for a given text file. Take some text and try to get the lexile measure for that text. It could be any RC passage, or any book (text format).
2) Read your text and see how difficult it is for you. May be score your comfort level on the scale of (easy) 1 to 10 (difficult).
3) tie your comfort level with the lexile measure. So, if you read a paragraph that is 1300L, and you felt it was 2, then you want to read books that are higher than 1300 (where your comfort level will drop a little). However, if you felt your comfort level was 9 for 1300L, you want to read books that are lower than 1300.
4) You may need to fine tune to get your level, and then practice at a little higher level.
5) now pickup a book that is little difficult for your comfort level, finish it, and then do the above exercise again. The website gives lexile measure for many books, pick the right one.

This way, if you read the content that is little higher, your chances of improvement are higher, and eventually you reach a level where you no longer feel challenged while answering RC.

Hope this helps. Your suggestions are welcome.
_________________

Some other posts by me...
My GMAT Journey
Timer Tool for Google spreadsheet
Quant Flash Cards/Pages
700 (Q:49 V:35) - Thanks GMATClub - Test Center experience