Unfortunately you will encounter low DSRs for a myriad of reasons ranging from buyers not knowing how the DSR system works, to being rated down due to the fact that the item didn't arrive when the buyer wanted it to (even if you post the same day as payment is received), to being rated a low DSR for postage even if you charge a buyer less than the actual cost of postage. Then of course DSRs can arise due to actions by the seller which is what they should be confined to in a perfect world. The irony is that ebay state that the DSRs are intended to let sellers know where they can improve yet they are anonymous and there is no recourse. eg even if you post within your specified handling time a buyer can leave a  rating of1 for postage time.

Over time you will encounter feedback and/or DSR ratings that you may feel are unfair or unwarranted. I think all sellers have experienced them. All you can do as a seller is provide great service and products to your buyers as the feedback and DSRs are effectively out of your control. As has been mentioned there are ways of identifying where the low DSRs came from if your sales volumes are not too high. You may want to block those buyers that you identify as leaving low DSRs.