I am trying to improve my performance in auditory backward retention tests. The task consists in trying to repeat in the reverse order the longest possible series of digits after hearing a series of up to 30 digits.

Is there a good way to tackle this test? More in general, is there a way to train working memory?

If it is a 30-digit number recited at once and you have to recall the longest range of subsequent digits you can recall in reverse order, it would be better to listen more carefully to the ones at the end, considering that it would much easier to recall a long string of numbers in reverse order from the end as you just heard it in comparison to the ones at the beginning which you are likely to forget by the time you reach the end of the thirty digit number. Now, you can increase the length of this string by grouping them together in packs of two or three, as @SerenaD mentioned. You can then reverse these numbers and answer accordingly. Another trick can be to find small patterns for every 3-4 digits, but this completely depends on how much time you get to analyze the numbers in this test, like at what speed do they read out the numbers and all. If it is slower, you will find it easier to form some correlations and remember these numbers better, if it is fast, the grouping strategy is your best hope. Practicing and playing memory games like finding the matching pair in cards and things like these can boost your memory, best of luck!!

Working memory has a span length of 7 +- 2 (Miller, 1956) digit. So I will suggest you remember the numbers as a bigger one. E.g if you hear 3, 8, 9, 4… you can pair them in 38, 94. This will mean memorizing only two numbers, so you will not "waste" your memory span.

This should be the trick, not sure if it works in practice. In theory, it does.

The brain is highly plastic. The more you train it the better will work.