I don't have any specific advice on how to do better on the shelves aside from studying hard using highly rated study resources and doing a ton of practice questions, but I'd like to put things in perspective for you:
1) your scores are not bad - they are at or above national average. Sure, one should always strive to do better but don't be fooled by people reporting 90+ scores on SDN - these are lean mean efficient study machines with 240+ 250+ Step scores - not everyone is quite as efficient, intelligent, hardworking, motivated etc as they are. And it's OK. I'm just saying that SDN is not representative of the general medical student population.
2) Speaking if Step scores - what's yours? You don't have to answer here, but my point is, if your Step 1 score is about average, it's not surprising that your shelf scores are just above average. And it's OK, too. Medical schools attract high academic achievers, and being average among them is no shame at all.