I had a lot of trouble with the distinction between dark and non-dark consonants, like Saad vs. Siin. At first, I only heard the difference in the following “aah” sound and no difference at all in the consonant. I sensitized my hearing by requesting Arabic native speakers on Rhinospike.com to record minimal pairs for me: each dark consonant combined with each of the vowels, and each non-dark equivalent consonant combined with each of the vowels, allowing a direct comparison. Audio here
The remaining problem was of course how to produce this distinction myself. My textbooks were supremely unhelpful, just talking about ‘darker’, ’emphatic’ or ‘pharyngealized’ sounds without telling me how to produce this effect. Finally I came across Michel Thomas’ Arabic course and it had good advice: the distinction between non-dark and dark S is close to the difference between the S in “see” or “sorry” in English. If you keep the tongue position from “sorry” and combine it with different vowel (like “ee”), you wind up with the Arabic dark S, or something very close to it. The other dark consonants can be acquired by assuming the same unusual tongue position.