If you come from a generally under-represented field (minority, LGBT or women), there are tons of opportunities to start networking the summer before your 1st year as an MBA student. Some of these are made possible by organizations (such as the Consortium for Graduate Study in Management, Toigo foundation, MBA Jumpstart, etc.). Others are held by banks themselves (Morgan Stanley, Goldman Sachs, JP Morgan, and UBS all hold pre-MBA camps ranging from 1-3 days where you can meet bankers and receive an insight into the industry). I'm not sure but i believe some consulting firms may also do something similar.