How do you change the default directory for saving/opening files? When saving my projects the default directory sometimes opens to C:\ but I have my blender projects saved in my D:\Blender Projects directory. Is there a way to make this directory the default so that I don't have to manually type it in every time I want to save something?

This would apply when opening files as well. My reference files are saved in a similar location D:\Blender Projects\References but to access them I have to type the D drive in manually because it default opens the C:\ drive every time.
You can't as Blender doesn't give us a way to do this. As gandalf3 says above in the comments it should use the last directory you accessed a file from in your current session. If you find yourself going to the same place several times, you can add a bookmark under Bookmarks or see your last visited folders under Recent in the sidebar. There should also be system bookmarks for your drives so no need to type if you can help it.