Before the Internship

i did a summer S&T at a BB, and while there are substantive differences between the two summer experiences - many things are still the same...i'd say one of the most important/beneficial things you can do is start reaching out to alumni that work at the firm and building up social capital now - say hi, introduce yourself, make them feel important by asking them if they have any advice, schedule lunch or drinks for the beginning of your summer, before things get crazy with work etc.

people get excited about summers coming in (at least they did at my bank) - there's a lot of chatter among young analysts along the lines of, oh so and so is in the summer class, the dude is sweet, i played xyz sport with him in HS. you want people to be on the look out for you specifically, if possible. it's a hell of a lot easier to meet expectations than it is impress people from scratch. if you can develop name recognition before you show up, it'll help you out, although again, i'm speaking from an S&T background, in which summers weren't really able to take on much responsibility/demonstrate aptitude and social capital was all you had. i'm sure other people will have more content driven answers for you.