Unfortunately there's no easy way to amass this type of knowledge, at least not that I've found. However, it often helps to categorize disease processes by type, to enable you to more easily generate differentials for any clinical problem you encounter. In other words, think vascular, infectious, neoplastic, degenerative, metabolic, autoimmune, etc. etc. There are some good mnemonics for these categories. Some people use "RICHMEN," "VINDICATE" or others, but I'm not a bit mnemonic person so I don't know what all those letters are supposed to be. I just think about really broad categories and it seems to work well for me.
This is also a great fall-back tool for when you get pimped and you don't really know the answer; think about possible processes, and rattle off some common players in each category.
In terms of remembering very specific differentials, such as the differential for macrocytic anemia or pancreatitis, I've found that it's just something that comes with time and experience. As a 2nd year I struggled to remember the major causes, but as time goes on you tend to broaden your differential knowledge base. The same goes with signs. It takes lots of time, exposure, and experience.
