Problem Solving- Backsolving strategies

If you'll know, after testing an answer choice and discovering it's wrong, whether the right answer should be larger or smaller, you should start with B or D. Say you start with B:

-if B is right, you're done;
-if B is too large, the answer is A, and you're done;
-if B is too small, move on to D. If D is right, you're done. If D is too large, then the answer is C and you're done. If D is too small, then the answer is E and you're done.

You only ever need to do at most two tests, and 2/5ths of the time you only need to do one test. If you start with C, you also only ever need to do two tests, but you'll always need to do a second test when C is the wrong answer - you can only get away with one test 1/5th of the time.

All of that said, knowing this is not likely to make much of a difference on your test. There aren't all that many questions you can backsolve in the first place, and fewer still on which you'll know, after testing a wrong answer, whether the correct answer is larger or smaller.

You may also read that, on questions which begin "which of the following...", it's best to start from answer choice E -- the idea being that the GMAT question designers assume people will start from A, and want to force people to do more work by trying out more answer choices. I've looked at as many official questions as I can find in this format, and there was no statistically significant difference in the frequency of each answer choice (C was actually the most common, though not by much), so I don't put much stock in that advice.